ASTRONOMY

QB43 i&

IX

POPULAR ASTRONOMY.

BY

SIMON NEWCOMB, LL.D.,

PROFESSOR U. S. NATAL OBSERVATORY, AUTHOR OF " THE A B C OF FINANCE."

With One Hundred and Ticdve Engraving*, and Five Maps of the Wars.

8vo. Oloth, $4 GO.

The grpo't reputation which the author of this work has merited and enjoys, both in this country and hi Europe, is a sufficient guarantee of its excellence. * * * He has dwelt especially upon those topics which have just now a popular and philosophic interest, carefully employing such lan- guage and such simple explanations as will be intelligible without labori- ous study. Technical terms have as much as possible been avoided. Such as were employed of necessity, and many that occur elsewhere, have been fully explained in a copious glossary at the end of the book. With its abundant aid, the reader cannot fail to derive both pleasure and entertain- ment from the study of what is the most ancient as well as the most ele- vating and inspiring of all the natural sciences. * * * Professor Newcomb, thro^hlWtrthi^^ltqfe' Volume, preserves his well-known character as a writer who, in treating of scientific subjects, fully understands the art of bringing them within the range of popular comprehension. Although his book is a valuable addition to scientific literature, it is fully calculated to hold the attention of the general reader. — N. Y. Times.

The problem of adapting the facts and principles of a most intricate science to the understanding of the ordinary reader has been earnestly undertaken and successfully solved in this work. * * * The entire volume bespeaks the well-known ability of its author, and furnishes a new title to his world-wide reputation. — Boston Transcript.

M298784

NewcomVs Popular Astronomy.

Professor Newcomb carefully avoids the temptation held out to him bj many parts of the subject to write for effect ; he keeps always faithfully to his purpose, setting forth, with respect to every subject discussed, the history of the investigations made, the positive, certain results attained, and the conjectures which astronomers have founded upon these results, together with the reasoning on which each conjecture rests and the objec- tions that exist to its acceptance. He is, in a word, singularly conscien- tious and perfectly frank ; but the subject itself is so full of wonders that even when treated in this calm, scientific spirit, its discussion is entran- cingly interesting; and Professor Newcomb's work, written as it is in a perfectly clear, simple, and direct style, > likely, we think, to become more than ordinarily popular. — N. Y. Evening Post.

The book has the great merit of a simplicity that never wearies the reader's attention. It presents the newest as well as the old discoveries, and is free from the errors which mar most of the treatises on astronomy that are designed for non-professional use. Ordinary readers will appre- ciate the circumstance that no mathematical formulas are employed. * * * In each division of the work the history of discovery is made to subserve the purpose of explanation. * * * Step by step the reader is led toward the theories of Copernicus, Kepler, and Newton, and is shown why and how their hypotheses best explained the facts of observation, which have been already detailed. A great advantage is thereby gained o^er ordinary trea- tises of astronomy, which present the recent knowledge first, and either give the facts unsupported, or press their acceptance by means of the stern logic of geometry. In Professor Newcomb's work the great truths grow slowly, and can be measured as they grow. — N. Y. Tribune.

The author is a master of all the theories and lore of his beloved sci- ence, and he has at command the unrivalled instruments of the United States Naval Observatory at Washington. He is an unwearied investiga- tor and professional enthusiast (in the best sense of the word), and writes an English which all people can understand. Parade and pedantry are wholly absent from this work. — jV. Y. Journal of Commerce.

Any person of average intelligence can take this volume, and in a month or two become an intelligent observer of the worlds around us. — Christian Intelligencer, N. Y.

NewcomUs Popular Astronomy.

This is one of those books which deserve and are sure to receive a hearty welcome : a full and accurate resume of the subject treated, prepared and brought down to date by one who is a master of the science, and at the same time a clear and vigorous writer. It is a book which ought to be in the library of every intelligent person as a standard authority, safely to be referred to on any topic within its scope ; and yet it is not heavy or dull, but, for the most part, as readable and interesting as a work of fiction. * * * The work is neither abstruse and dry, nor, on the other hand, is it puerile and fanciful, as sometimes happens when savans attempt to popu- larize their favorite sciences, and write down to what they conceive to be the level of the common intelligence. The plan is logical, the due propor- tions of different portions of the subject are observed, and the style is clear, forcible, and sufficiently picturesque and stimulating to keep the attention without effort. — Professor CHARLES A. YOCNG, in tfie Independent, N. Y.

It is only rarely that a great mathematical astronomer condescends to write books for the people ; and if he does, in four cases out of five, what he writes is unintelligible to all but a very few. Investigators seldom have either the disposition or ability to communicate what they know to the world in general. To this rule, however, there are happy exceptions ; and among them must be counted Professor Xewcomb, whose Popular Astron- omy is undoubtedly the best work of its kind in the English language. Its arrangement is logical, its statements are accurate, its reasonings clear, and its style simple, perspicuous, and sufficiently picturesque. Through- out the book it is everywhere evident that great care has been taken to secure exact and perfect truthfulness of representation: facts are kept distinct from fancies, and theories and speculations stand for just what they are. — Sunday School Times, Philadelphia.

PUBLISHED BY HARPER & BROTHERS, NEW YORK.

Sent by mail, postage prepaid, to any part of tJie Untied States, on receipt of $4 00.

THI ¥YXHI

THI AFAnHTHI

ASTPACTOYSHI

KAI

ISAITEAQI

PREFACE.

ALL sciences are making an advance, but Astronomy is moving at the double-quick. Since the principles of this science were settled by Copernicus, four hundred years ago, it has never had to beat a retreat. It is re- written not to correct material errors, but to incorporate new discoveries.

Once Astronomy treated mostly of tides, seasons, and telescopic aspects of the planets; now these are only primary matters. Once it considered stars as mere fixed points of light ; now it studies them as suns, determines their age, size, color, movements, chemical constitution, and the revolution of their planets. Once it considered space as empty ; now it knows that every cubic inch of it quivers with greater intensity of force than that which is visible in Niagara. Every inch of surface that can be conceived of between suns is more wave-tossed than the ocean in a storm.

The invention of the telescope constituted one era in Astronomy ; its perfection in our day, another ; and the discoveries of the spectroscope a third — no less impor- tant than either of the others.

While nearly all men are prevented from practical experimentation in these high realms of knowledge, few

viii PREPACK

have so little leisure as to be debarred from intelligently enjoying the results of the investigations of others.

This book has been written not only to reveal some of the highest achievements of the human mind, but also to let the heavens declare the glory of the Divine Mind. In the author's judgment, there is no gulf that separates science and religion, nor any conflict where they stand together. And it is fervently hoped that any one who comes to a better knowledge of God's works through reading this book, may thereby come to a more intimate knowledge of the Worker.

I take great pleasure in acknowledging my indebted- ness to J. M. Van Yleck, LL.D., of the U. S. Nautical Almanac staff, and Professor of Astronomy at the Wes- leyan University, for inspecting some of the more im- portant chapters ; to Dr. S. S. White, of Philadelphia, for telescopic advantages ; to Professor Henry Draper, for furnishing, in advance of publication, a photograph of the sun's corona in 1878 ; and to the excellent work on "Popular Astronomy," by Professor Simon New- comb, LL.D., Professor U. S. Naval Observatory, for some of the most recent information, and for the use of the unequalled engravings of Jupiter, Saturn, and the great nebula of Orion.

THE CONSTELLATIONS

ORION AND TAURUS,

NOTES.— Star a in Taurus is red, has eight metals ; moves east (page 22T). At o above tip of right horn is the Crab Nebula (page 219). In Orion, a is variable, hao five metals ; recedes 22 miles per second. /3, 3, e, f , p, etc., are double stars, the component parts of various colors and magnitudes (page 212, note). X and « are triple ; <r, octuple; 0, multiple, surrounded by a fine Nebula (page 218).

RECREATIONS IN ASTRONOMY

WITH

DIRECTIONS FOR PRACTICAL EXPERIMENTS AND TELESCOPIC WORK

BY

HEXRY WHITE WARREK, D.D.

AUTHOR OF "SIGHTS AND INSIGHTS; OR, KNOWLEDGE BY TRAVEL," ETC.

WITH EIGHTY-THREE ILLUSTRATIONS AND MAPS OF STARS

NEW YORK

HARPER & BROTHERS, PUBLISHERS

FRANKLIN SQUARE

Entered according to Act of Congress, in the year 1879, by

HARPER & BROTHERS, In the Office of the Librarian of Congress, at Washington,

CONTENTS.

CHAP. PAGE

I. CREATIVE PROCESSES 1

II. CREATIVE PROGRESS 15

Constitution of Light 24

Chemistry of Suns revealed by Light 28

Creative Force of Light 30

III. ASTRONOMICAL INSTRUMENTS 41

The Telescope 43

The Reflecting Telescope 44

The Spectroscope 46

IV. CELESTIAL MEASUREMENTS 55

Celestial Movements 58

How to Measure 60

V. THE SUN 75

What the Sun does for us 94

VI. THE PLANETS, AS SEEN FROM SPACE 97

The Outlook from the Earth 108

VII. SHOOTING-STARS, METEORS, AND COMETS 117

Aerolites 122

Comets 126

Famous Comets 128

Of what do Comets consist? 131

Will Comets strike the Earth ? 133

VIII. THE PLANETS AS INDIVIDUALS 135

Vulcan 138

Mercury 138

Venus 139

The Earth 141

The Aurora Borealis 143

CONTENTS.

CHAP.

VIII. THE PLANETS AS INDIVIDUALS— Continued.

The Delicate Balance of Forces ................................... 144

Tides ..................................................................... 146

The Moon ............................................................... 151

Telescopic Appearance ............................................... 155

Eclipses .................................................................. 157

Mars ...................................................................... 159

Satellites of Mars ...................................................... 161

Asteroids ................................................................ 162

Jupiter ................... ................................................ 164

Satellites of Jupiter ................................................... 165

Saturn .................................................................... 167

Rings of Saturn ........................................................ 169

Satellites of Saturn .................................................... 172

Uranus. .................................................................. 173

Neptune ................................................................. 175

IX. THE NEBULAR HYPOTHESIS .......................................... 179

X. THE STELLAR SYSTEM ................................................. 193

The Open Page of the Heavens .................................... 195

Equatorial Constellations ............................................ 202

Characteristics of the Stars .......................................... 209

Number .................................................................. 210

Double and Multiple Stars .......................................... 210

Colored Stars ........................................................... 214

Clusters of Stars ........ , .............................................. 215

Nebulae ................................................................... 217

Variable Stars ......................................................... 220

Temporary, New, and Lost Stars .................................. 223

Movements of Stars ................................................... 226

XI. THE WORLDS AND THE WORD ..................................... 229

XII. THE ULTIMATE FORCE ................................................ 247

SUMMARY OF LATEST DISCOVERIES AND CONCLUSIONS ............... 268

SOME ELEMENTS or THE SOLAR SYSTEM ................................. 274

EXPLANATION OF ASTRONOMICAL SYMBOLS ............................... 275

Signs of the Zodiac ................................................... 275

Other Abbreviations Used in the Almanac ..... ................. 275

Greek Alphabet Used Indicating the Stars ...................... 275

CHAUTAUQUA OUTLINE FOK STUDENTS ..................................... 276

GLOSSARY OF ASTRONOMICAL TERMS AND INDEX .............. , ....... 279

ILLUSTRATIONS.

FIG. PAGE

The Constellations of Orion and Taurus Frontispiece

1. An Orbit resulting from Attraction and Projection 8

2. The Moon's Orbit about the Earth 10

3. Changes of Orbit by Mutual Attraction 11

4. Velocity of Light measured by Jupiter's Satellites 22

5. Velocity of Light measured by Fizeau's Toothed Wheel 23

6. White Light resolved into Colors 25

7. Showing amount of Light received by Different Planets 37

8. Measuring Intensities of Lights 37

9. Reflection and Diffusion of Light 38

10. Manifold Reflections 39

11. Refraction by Water 40

12. Atmospherical Refraction 40

13. Refracting Telescope 43

14. Reflecting Telescope 44

15. The Cambridge Equatorial Refractor 46

16. The new Reflecting Telescope at Paris 47

17. Spectroscope, with Battery of Prisms 49

18. Spectra of Glowing Hydrogen and of the Sun 50

19. Illustrating Arcs and Angles 59

20. Measuring Objects by observing Angles ."»9

21. Mural Circle 61

22. Scale to measure Hundredths of an Inch 63

23. Spider-lines to determine Star Transits 65

24. Illustrating Triangulation 66

xii ILLUSTRATIONS.

FIG. PAGE

25. Measuring Distance to an Inaccessible Object 67

26. Measuring Elevation of an Inaccessible Object 67

27. Illustrating Parallax 69

28. Illustrating Stellar Parallax 71

29. Mode of Ascertaining Longitude 72

30. Relative Size of Sun, as seen from Different Planets 79

31. Zodiacal Light 80

32. Corona of the Sun in 1858— Brazil 82

33. Corona of the Sun in 1878— Colorado 83

34. Solar Prominences of Flaming Hydrogen 85

35. Changes in Solar Cavities during Rotation 90

36. Solar Spot 92

37. Holding Telescope to see the Sun-spots 96

38. Orbits and Comparative Sizes of the Planets 100

39. Orbit of Earth, illustrating Seasons 103

40. Inclination of Planes of Planetary Orbits 107

41. Inclination of Orbits of Earth and Venus 107

42. Showing the Sun's Movement among the Stars 110

43. Passage of the Sun by Star Regulus Ill

44. Apparent Path of Jupiter among the Stars 112

45. Illustrating Position of Planets 112

46. Apparent Movements of an Inferior Planet. 113

47. Apparent Movements of a Superior Planet 114

47a. A Swarm of Meteors meeting the Earth 118

48. Explosion of a Bolide 120

49. Flight of Bolides 121

50. The Santa Rosa Aerolite 122

51. Orbit of November Meteors and the Comet of 1866 125

52. Aspects of Remarkable Comets 127

53. Phases and Apparent Dimensions of Venus 140

54. The Earth and Moon in Space 142

55. Aurora as Waving Curtains 143

56. Tide resulting from Centrifugal Motion 147

57. Lunar Landscape , 150

ILLUSTRATIONS. xiii

FIG. PAGE

58. Telescopic View of the Moon 154

59. Illumination of Lunar Craters and Peaks 155

60. Lunar Crater "Copernicus" 156

61. Eclipses: Shadows of Earth and Moon 157

62. Apparent Sizes of Mars, seen from the Earth 160

63. Jupiter 164

64. Various Positions of Jupiter's Satellites 166

65. View of Saturn and his Rings 168

66. Perturbations of Uranus 176

67. Map: Circumpolar Constellations 201

68. Map of Constellations on the Meridian in December 202

.69. Map of Constellations on the Meridian in January 203

70. Map of Constellations on the Meridian in April 204

71. Map of Constellations on the Meridian in June 205

72. Map of Constellations on the Meridian in September.... 206

73. Map of Constellations on the Meridian in November. 207

74. Southern Circumpolar Constellations 208

75. Aspects of Double Stars 213

76. Sprayed Star Cluster below i\ in Hercules 216

77. Globular Star Cluster in the Centaur 216

78. Great Nebula about 6 Orionis 218

79. The Crab Nebula above £ Tauri 219

80. The Ring Nebula in Lyra 220

81. Showing Place of Ring Nebula 221

82. The Horizontal Pendulum .. 272

COLORED PLATE REPRESENTING VARIOUS SPECTRA 50-52

MAPS TO FIND THE STARS .. At the End.

I.

CREATIVE PROCESSES.

" In the beginning God created the heaven and the earth. And the earth was without form, and void ; and darkness was upon the face of the deep." — Genesis i. 1,2.

1

"Not to the domes, where crumbling arch and column

Attest the feebleness of mortal hand, But to that fane, most catholic and solemn,

Which God hath planned, — To that cathedral, boundless as our wonder,

Whose quenchless lamps the sun and moon supply , Its chdr the winds and waves, its organ thunder,

Its dome the sky." HOHACK SMITH.

' The heavens are a point from the pen of His perfection ; The world is a rose-bud from the bower of His beauty ; The sun is a spark from the light of His wisdom ; And the sky a bubble on the sea of His power."

SIR W. JOXES.

RECREATIONS IN ASTRONOMY.

I.

CREATIVE PROCESSES.

DURING all the ages there has been one bright and glittering page of loftiest wisdom unrolled before the eye of man. That this page may be read in every part, man's whole world turns him before it. This motion apparently changes the eternally stable stars into a mov- ing panorama, but it is only so in appearance. The sky is a vast, immovable dial-plate of " that clock whose pendulum ticks ages instead of seconds," and whose time is eternity. The moon moves among the illumi- nated figures, traversing the dial quickly, like a second- hand, once a month. The sun, like a minute-hand, goes over the dial once a year. Various planets stand for hour-hands, moving over the dial in various periods reaching up to one hundred and sixty-four years ; while the earth, like a ship of exploration, sails the infinite azure, bearing the observers to different points where they may investigate the infinite problems of this mighty machinery.

This dial not only shows present movements, but it keeps the history of uncounted ages past ready to be

4 CREATIVE PROCESSES.

read backward in proper order ; and it has glorious vol- umes of prophecy, revealing the far-off future to any man who is able to look thereon, break the seals, and read the record. Glowing stars are the alphabet of this lofty page. They combine to form words. Mete- ors, rainbows, auroras, shifting groups of stars, make pictures vast and significant as the armies, angels, and falling stars in the Revelation of St. John — changing and progressive pictures of infinite wisdom and power.

Men have not yet advanced as far as those who saw the pictures John describes, and hence the panorama is not understood. That continuous speech that day af- ter day uttereth is not heard ; the knowledge that night after night showeth is not seen ; and the invisible things of God from the creation of the world, even his eternal power and Godhead, clearly discoverable from things that are made, are not apprehended.

The greatest triumphs of men's minds have been in astronomy — and ever must be. We have not learned its alphabet yet. We read only easy lessons, with as many mistakes as -happy guesses. But in time we shall know all the letters, become familiar with the combi- nations, be apt at their interpretation, and will read with facility the lessons of wisdom and power that are written on the earth, blazoned in the skies, and pictured by the flowers below and the rainbows above.

In order to know how worlds move and develop, we must create them ; we must go back to their begin- ning, give their endowment of forces, and study the laws of their unfolding. This we can easily do by that faculty wherein man is likest his Father, a creative im- agination. God creates and embodies; we create, but

FORCE OF ATTRACTION. 5

it remains in thought only. But the creation is as bright, strong, clear, enduring, and real, as if it were em- bodied. E\7ery one of us would make worlds enough to crush us, if we could embody as well as create. Our ambition would outrun our wisdom. Let us come into the high and ecstatic frame of mind which Shakspeare calls frenzy, in the exigencies of his verse, when

"The poet's eye, in a fine frenzy rolling, Doth glance from heaven to earth, from earth to heaven ; And, as imagination bodies forth The forms of things unknown, the poet's pen Turns them to shapes, and gives to airy nothing A local habitation and a name."

la the supremacy of our creative imagination let us make empty space, in order that we may therein build up a new universe. Let us wave the wand of our pow- er, so that all created things disappear. There is no world under our feet, no radiant clouds, no blazing sun, no silver moon, nor twinkling stars. We look up, there is no light ; down, through immeasurable abysses, there is no form; all about, and there is no sound or sign of being — nothing save utter silence, utter darkness. It cannot be endured. Creation is a necessity of mind — even of the Divine mind.

We will now, by imagination, create a monster world, every atom of which shall be dowered with the single power of attraction. Every particle shall reach out its friendly hand, and there shall be a drawing to- gether of every particle in existence. The laws gov- erning this attraction shall be two. When these parti- cles are associated together, the attraction shall be in proportion to the mass. A given mass will pull twice

G CREATIVE PROCESSES.

as much as one of half the size, because there is twice as much to pull. And a given mass will be pulled twice as much as one half as large, because there is twice as much to be pulled. A man who weighed one hundred and fifty pounds on the earth might weigh a ton and a half on a body as large as the sun. That shall be one law of attraction; and the other shall be that masses attract inversely as the square of distances between them. Absence shall affect friendships that have a material basis. If a body like the earth pulls a man one hundred and fifty pounds at the surface, or four thousand miles from the centre, it will pull the same man one -fourth as much at twice the distance, one-sixteenth as much at four times the distance. That is, he will weigh by a spring balance thirty-seven and a half pounds at eight thousand miles from the centre, and nine pounds six ounces at sixteen thousand miles from the centre, and he will weigh or be pulled by the earth ¥V of a pound at the distance of the moon. But the moon would be large enough and near enough to pull twenty-four pounds on the same man, so the earth could not draw him away. Thus the two laws of at- traction of gravitation are — 1, Gravity is proportioned to the quantity of matter ; and 2, The force of gravity varies inversely as the square of the distance from the centre of the attracting l)ody.

The original form of matter is gas. Almost as I write comes the announcement that Mr. Lockyer has proved that all the so-called primary elements of mat- ter are only so many different sized molecules of one original substance — hydrogen. Whether that is true or not, let us now create all the hydrogen we can

FORCE OF ATTRACTION. 7

imagine, either in differently sized masses or in com- bination with other substances. There it is ! We can- not measure its bulk ; we cannot fly around it in any recordable eons of time. It has boundaries, to be sure, for we are finite, but we cannot measure them. Let it alone, now ; leave it to itself. What follows ? It is dowered simply with attraction. The vast mass begins to shrink, the outer portions are drawn inward. They rush and swirl in vast cyclones, thousands of miles in extent. The centre grows compact, heat is evolved by impact, as will be explained in Chapter II. Dull red light begins to look like coming dawn. Centuries go by ; contraction goes on ; light blazes in insufferable brightness ; tornadoes, whirlpools, and tempests scarcely signify anything as applied to such tumultuous tossing.

There hangs the only world in existence ; it hangs in empty space. It has no tendency to rise ; none to fall ; none to move at all in any direction. It seethes and flames, and holds itself together by attractive power, and that is all the force with which we have endowed it.

Leave it there alone, and withdraw millions of miles into space: it looks smaller and smaller. We lose sight of those distinctive spires of flame, those terrible move- ments. It only gives an even effulgence, a steady un- flickering light. Turn one quarter round. Still we see our world, but it is at one side.

Now in front, in the utter darkness, suddenly create another world of the same size, and at the same distance from you. There they stand — two huge, lone bodies, in £mpty space. But we created them dowered with at- traction. Each instantly feels the drawing influence of the other. They are mutually attractive, and begin to

8 CREATIVE PROCESSES.

move toward each other. They hasten along an uncle- viating straight line. Their speed quickens at every mile. The attraction increases every moment. They fly swift as thought. They clash their flaming, seething foreheads together.

And now we have one world again. It is twice as large as before, that is all the difference. There is no variety, neither any motion ; just simple flame, and noth- ing to be warmed thereby. Are our creative powers exhausted by this effort ?

No, we will create another world, and add another power to it that shall keep them apart. That power

D

Fig. 1.— Orbit A D, resultiug from attraction, A C, and projectile force, A B.

shall be what is called the force of inertia, which is literally no power at all ; it is an inability to originate or change motion. If a body is at rest, inertia is that quality by which it will forever remain so, unless acted upon by some force from without ; and if a body is in motion, it will continue on at the same speed, in a straight line, forever, unless it is quickened, retarded, or turned from its path by some other force. Suppose our newly created sun is 860,000 miles in diameter. Go away 92,500,000 miles and create an earth eight thou- sand miles in diameter. It instantly feels the at- tractive power of the sun drawing it to itself twenty-

FORCES OF ATTRACTION AND INERTIA. 9

four miles the first hour. Now, just as it starts, give this earth a push in a line at right angles with line of fall to the sun, that shall send it 66,168 miles every hour thereafter. It obeys both forces. The result is that the world moves constantly forward at the same speed by its inertia from that first push, and attraction momentarily draws it from its straight line, so that the new world circles round the other to the starting-point. Continuing under the operation of both forces, the worlds can never come together or fly apart.

They circle about each other as long as these forces endure ; for the first world does not stand still and the second do all the going ; both revolve around the centre of gravity common to both. In case the worlds are equal in mass, they will both take the same orbit around a central stationary point, midway between the two. In case their mass be as one to eighty-one, as in the case of the earth and the moon, the centre of grav- ity around which both turn will be -gV of the distance from the earth's centre to the moon's centre. This brings the central point around which both worlds swing just inside the surface of the earth. It is like an apple attached by a string, and swung around the hand ; the hand moves a little, the apple very much.

Thus the problem of two revolving bodies is readily comprehended. The two bodies lie in easy beds, and swing obedient to constant forces. "When another body, however, is introduced, with its varying attraction, first on one and then on the other, complications are intro- duced that only the most masterly minds can follow. Introduce a dozen or a million bodies, and complica- tions arise that only Omniscience can unravel.

1*

10 CREATIVE PROCESSES.

Let the hand swing an apple by an elastic cord. When the apple falls toward the earth it feels another force besides that derived from the hand, which greatly lengthens the elastic cord. To tear it away from the earth's attraction, and make it rise, requires additional force, and hence the string is lengthened ; but when it passes over the hand the earth attracts it downward, and the string is very much shortened : so the moon, held by an elastic cord, swings around the earth. From its extreme distance from the earth, at A, Fig. 2, it rushes with increasing speed nearly a

O quarter of a million of miles toward c the sun, feeling its attraction increase with every mile until it reaches B; then it is retarded in its speed, by the same attraction, as it climbs back its quarter of a million of miles away from the sun, in defiance of its pow- er, to C. All the while the invisi- ble elastic force of the earth is un- weariedly maintained ; and though the moon's dis- tances vary over a range of 31,355 miles, the moon is always in a determinable place. A simple revolu- tion of one world about another in a circular orbit would be a problem of easy solution. It would always be at the same distance from its centre, and going with the same velocity. But there are over sixty causes that interfere with such a simple orbit in the case of the moon, all of which causes and their disturbances must be considered in calculating such a simple matter as an eclipse, or predicting the moon's place as the sailors' guide. One of the most puzzling of the irregularities

FORCES OF ATTRACTION AND INERTIA. 11

of our night-wandering orb has just been explained by Professor Hansen, of Gotha, as a curious result of the attraction of Venus.

Take a single instance of the perturbations of Jupiter and Saturn which can be rendered evident. The times of orbital revolution of Saturn and Jupiter are nearly as five to two. Suppose the orbits of the planets to be, as in Fig. 3, both ellipses, but not necessarily equally distant in all parts. The planets are as near as possible at 1,1. Drawn toward each oth- er by mutual attraction, Jupi- ter's orbit bends outward, and Fig.3._c^f^f orbit by Saturn's becomes more nearly mutual attraction, straight, as shown by the dotted lines. A partial cor- rection of this difficulty immediately follows. As Jupi- ter moves on ahead of Saturn it is held back — retarded in its orbit by that body ; and Saturn is hastened in its orbit by the attraction of Jupiter. Now greater speed means a straighter orbit. A rifle-ball flies nearer in a straight line than a thrown stone. A greater velocity given to a whirled ball pulls the elastic cord far enough to give the ball a larger orbit. Hence, being hastened, Saturn stretches out nearer its proper orbit, and, retard- ed, Jupiter approaches the smaller curve that is its true orbit.

But if they were always to meet at this point, as they would if Jupiter made two revolutions to Saturn's one, it would be disastrous. In reality, when Saturn has gone around two-thirds of its orbit to 2, Jupiter will have gone once and two-thirds around and overtaken

12 CREATIVE PROCESSES.

Saturn ; and they will be near again, be drawn togeth- er, hastened, and retarded, as before ; their next con- junction would be at 3, 3, etc.

Now, if they always made their conjunction at points equally distant, or at thirds of their orbits, it would cause a series of increasing deviations; for Jupiter would be constantly swelling his orbit at three points, and Saturn increasingly contracting his orbit at the same points. Disaster would be easily foretold. But as their times of orbital revolutions are not exactly in the ratio of five and two, their points of conjunction slowly travel around the orbit, till, in a period of nine hundred years, the starting-point is again reached, and the perturba- tions have mutually corrected one another.

For example, the total attractive effect of one planet on the other for 450 years is to quicken its speed. The effect for the next 450 years is to retard. The place of Saturn, when all the retardations have accumulated for 450 years, is one degree behind what it is computed if they are not considered ; and 450 years later it wrill be one degree before its computed place — a perturbation of two degrees. When a bullet is a little heavier or ragged on one side, it will constantly swerve in that di- rection. The spiral groove in the rifle, of one turn in forty-five feet, turns the disturbing weight or raggedness from side to side — makes one error correct another, and so the ball flies straight to the bull's-eye. So the place of Jupiter and Saturn, though further complicated by four moons in the case of Jupiter, and eight in the case of Saturn, and also by perturbations caused by other plan- ets, can be calculated with exceeding nicety.

The difficulties would be greatly increased if the or*

FORCES OF ATTRACTION AND INERTIA. 13

bits of Saturn and Jupiter, instead of being 400,000,000 miles apart, were interlaced. Yet there are the orbits of one hundred and ninety-two asteroids so interlaced that, if they were made of wire, no one could be lifted without raising the whole net-work of them. Never- theless, all these swift chariots of the sky race along the course of their intermingling tracks as securely as if they were each guided by an intelligent mind. They are guided Try an intelligent tnind and an almighty arm.

Still more complicated is the question of the mutual attractions of all the planets. Lagrange has been able to show, by a mathematical genius that seems little short of omniscience in his single department of knowledge, that there is a discovered system of oscillations, affect- ing the entire planetary system, the periods of which are immensely long. The number of these oscillations is equal to that of all the planets, arid their periods range from 50,000 to 2,000,000 years.

Looking into the open page of the starry heavens wo see double stars, the constituent parts of which must re- volve around a centre common to them both, or rush to a common ruin. Eagerly we look to see if they revolve, and beholding them in the. very act, we conclude, not groundlessly, that the same great law of gravitation holds good in distant stellar spaces, and that there the same sufficient mind plans, and the same sufficient power directs and controls all movements in harmony and security.

When we come to the perturbations caused by the mutual attractions of the sun, nine planets, twenty moons, one hundred and ninety-two asteroids, millions

14: CREATIVE PROCESSES.

of comets, and innumerable meteoric bodies swarming in space, and when we add to all these, that belong to one solar system, the attractions of all the systems of the other suns that sparkle on a brilliant winter night, we are compelled to say, " As high as the heavens are above the earth, so high above our thoughts and ways must be the thoughts and ways of Him who compre- hends and directs them all."

II.

CREATIVE PROGRESS,

"And God said, Let there be light, and there was light." — Genesis i., 3.

" God is light."— 1 John, i., 5.

" Hail! holy light, offspring of Heaven first born, Or of the eternal, co-eternal beam, May I express thee unblamed ? since God is light, And never but in unapproached light Dwelt from eternity, dwelt then in thee, Bright effluence of bright essence increate."

MILTON.

" A million torches lighted by Thy hand Wander unwearied through the blue abyss : They own Thy power, accomplish Thy command, All gay with life, all eloquent with bliss. What shall we call them ? Piles of crystal light— A glorious company of golden streams — Lamps of celestial ether burning bright — Suns lighting systems with their joyous beams ? But Thou to these art as the noon to night."

DERZHAVIN, trans, by BOWRING,

FORCES OF THE SUNBEAM. 17

II. CREATIVE PROGRESS.

WORLDS would be very imperfect and useless when simply endowed with attraction and inertia, if no time were allowed for these forces to work out their legiti- mate results. We want something more than swirling seas of attracted gases, something more than compacted rocks. We look for soil, verdure, a paradise of beauty, animal life, and immortal minds. Let us go on with the process.

Light is the child of force, and the child, like its fa- ther, is full of power. We dowered our created world with but a single quality — a force of attraction. It not only had attraction for its own material substance, but sent out an all-pervasive attraction into space. By the force of condensation it flamed like a sun, and not only lighted its own substance, but it filled all space with the luminous outgoings of its power. A world may be limited, but its influence cannot ; its body may have bounds, but its soul is infinite. Everywhere is its mani- festation as real, power as effective, presence as actual, as at the central point. He that studies ponderable bodies alone is not studying the universe, only its skel- eton. Skeletons are somewhat interesting in themselves, but far more so when covered with flesh, flushed with beauty, and inspired with soul. The universe has bones,

18 CREATIVE PROGRESS.

flesh, beauty, soul, and all is one. It can be understood only by a study of all its parts, and by tracing effect to cause.

But how can condensation cause light? Power can- not be quiet. The mighty locomotive trembles with its own energy. A smitten piece of iron has all its infini- tesimal atoms set in vehement commotion ; they surge back and forth among themselves, like the waves of a storm-blown lake. Heat is a mode of motion. A heat- ed body commences a vigorous vibration among its par- ticles, and communicates these vibrations to the sur- rounding air and ether. "When these vibrations reach 396,000,000,000,000 per second, the human eye, fitted to be affected by that number, discerns the emitted un- dulations, and the object seems to glow with a dull red light ; becoming hotter, the vibrations increase in rapid- ity. When they reach 765,000,000,000,000 per second the color becomes violet, and the eye can observe them no farther. Between these numbers are those of differ- ent rapidities, which affect the eye — as orange, yellow, green, blue, indigo, in an almost infinite number of shades — according to the sensitiveness of the eye.

We now see how our dark immensity of attractive atoms can become luminous. A force of compression results in vibrations within, communicated to the ether, discerned by the eye. Illustrations are numerous. If we suddenly push a piston into a cylinder of brass, the force produces heat enough to set fire to an inflam- mable substance within. Strike a half -inch cube of iron a moderate blow and it becomes warm ; a sufficient blow, and its vibrations become quick enough to be seen — it is red-hot. Attach a thermometer to an extended

FORCES OF THE SUNBEAM. 19

arm of a whirling wheel ; drive it against the air five hundred feet per second, the mercury rises 16°. The earth goes 98,000 feet per second, or one thousand miles a minute. If it come to an aerolite or mass of metallic rock, or even a cloudlet of gas, standing still in space, its contact with our air evolves 600,000° of heat. And when the meteor comes toward the world twenty -six miles a second, the heat would become proportionally greater if the meteor could abide it, and not be con- sumed in fervent heat. It vanishes almost as soon as seen. If there were meteoric masses enough lying in our path, our sky would blaze with myriads of flashes of light. Enough have been seen to enable a person to read by them at night. If a sufficient number were present, we should miss their individual flashes as they blend their separate fires in one sea of insufferable glory. The sun is 326,800 as heavy as our planet ; its attraction proportionally greater; the aerolites more numerous; and hence an infinite hail of stones, small masses and little worlds, makes ceaseless trails of light, whose individuality is lost in one dazzling sea of glory? On the 1st day of September, 1859, two astronomers, independently of each other, saw a sudden brightening on the surface of the sun. Probably two large meteoric masses were travelling side by side at two or three hun- dred miles per second, and striking the sun's atmos- phere, suddenly blazed into light bright enough to be seen on the intolerable light of the photosphere as a background. The earth responded to this new cause of brilliance and heat in the sun. Yivid auroras ap- peared, not only at the north and south poles, but even where such spectacles are seldom seen. The electro-

20 CREATIVE PROGRESS.

magnetic disturbances were more distinctly marked. " In many places the telegraphic wires struck work. In Washington and Philadelphia the electric signalmen re- ceived severe electric shocks; at a station in Norway, the telegraphic apparatus was set fire to ; and at Boston a flame of tire followed the pen of Bain's electric tele- graph." There is the best of reason for believing that a continuous succession of such bodies might have gono far toward rendering the earth uncomfortable as a place of residence.

Of course, the same result of heat and light would follow from compression, if a body had the power of contraction in itself. We endowed every particle of our gas, myriads of miles in extent, with an attraction for every other particle. It immediately compressed itself into a light-giving body, which flamed out through the interstellar spaces, flushing all the celestial regions with exuberant light.

But heat exerts a repellent force among particles, and soon an equilibrium is reached, for there comes a time when the contracting body can contract no farther. But heat and light radiate away into cold space, then contrac- tion goes on evolving more light, and so the suns flame on through the millions of years unquenched. It is es- timated that the contraction of our sun, from filling im- mensity of space to its present size, could not afford heat enough to last more than 18,000,000 years, and that its contraction from its present density (that of a swamp) to such rock as that of which our earth is composed, could supply heat enough for 17,000,000 years longer. But the far-seeing mind of man knows a time must come when the present force of attraction

FORCES OF THE SUNBEAM. 21

shall liave produced all the heat it can, and a new force of attraction must be added, or the sun itself will be- come cold as a cinder, dead as a burned-out char.

Since light and heat are the product of such enor- mous cosmic forces, they must partake of their nature, and be force. So they are. The sun has long arms, and they are full of unconquerable strength ninety-two millions, or any other number of millions, of miles away. All this light and heat comes through space that is 200° below zero, through utter darkness, and ap- pears only on the earth. So the gas is darkness in the underground pipes, but light at the burner. So the electric power is unfelt by the cable in the bosom of the deep, but is expressive of thought and feeling at the end. Having found the cause of light, we will com- mence a study of its qualities and powers.

Light is the astronomer's necessity. "When the sub- lime word was uttered, " Let there be light !" the study of astronomy was made possible. Man can gather but little of it with his eye ; so he takes a lens twenty-six inches in diameter, and bends all the light that passes through it to a focus, then magnifies the image and takes it into his eye. Or he takes a mirror, six feet in diameter, so hollowed in the middle as to reflect all the rays falling upon it to one point, and makes this larger eye fill his own with light. By this larger light-gath- ering he discerns things for which the light falling on his pupil one-fifth of an inch in diameter would not be sufficient. We never have seen any sun or stars; we have only seen the light that left- them a few min- utes or years ago, more or less. Light is the aerial sprite that carries our measuring-rods across the infinite

22 CREATIVE PROGRESS.

spaces ; light spreads out the history of that far-off be- ginning ; brings us the measure of stars a thousand times brighter than our sun ; takes up into itself evidences of the very constitutional elements of the far-off suns, and spreads them at our feet. It is of such capacity that the Divine nature, looking for an expression of its own omnipotence, omniscience, and power of revelation, was content to say, " God is Light." We shall need all our delicacy of analysis and measurement when we seek to determine the activities of matter so fine and near to spirit as light.

We first seek the velocity of light. In Fig. 4 the earth is 92,500,000 miles from the sun at E; Jupiter is 480,000,000 miles from the sun at J. It has four

Fig. 4.— Velocity of Light measured by Eclipses of Jupiter's Moons.

moons : the inner one goes around the central body in forty-two hours, and is eclipsed at every revolution. The light that went out from the sun to M ceases to be reflected back to the earth by the intervention of the plan- et Jupiter. We know to a second when these eclipses take place, and they can be seen witli a small telescope. But when the earth is on the opposite side of the sun

FORCES OF THE SUNBEAM.

23

from Jupiter, at E', these eclipses at J' take place sixteen and a half minutes too late. What is the reason? Is the celestial chronometry getting deranged ? No, indeed ; these great worlds swing never an inch out of place, nor a second out of time. By going to the other side of the sun the earth is 18^,000,000 miles farther from Jupiter, and the light that brings the intelligence of that eclipse consumes the extra time in going over the extra distance. Divide one by the other and we get the velocity, 185,000 miles per second. That is probably correct to within a thousand miles. Methods of measurement by the toothed wheel of Fizeau confirm this re- sult. Suppose the wheel, Fig. 5, to have one thou- sand teeth, making five revolutions to the second. Five thousand flashes of light each second will

dart out. Let each flash travel nine miles to a mirror and return. If it goes that distance in i0ioo of a sec- ond, or at the rate of 180,000 miles a second, the next tooth will have arrived before the eye, and each return- ing ray be cut off. Hasten the revolutions a little, and the next notch will then admit the ray, on its return, that went out of each previous notch : the eighteen miles hav- ing been traversed meanwhile. The result of experi- ments by Lieut. Michelson, as given Sept., 1879, is 186,305 miles per second, which cannot be fifty miles from correct. When we take instantaneous photographs by the ex-

Fig. 5.— Measuring the Velocity of Light.

24: CREATIVE PROGRESS.

posure of the sensitive plate -jimnr Pai*t of a second, a stream of light nine miles long dashes in upon the plate in that very brief period of time.

The highest velocity we can give a rifle-ball is 2000 feet a second, the next second it is only 1500 feet, and soon it comes to rest. We cannot compact force enough behind a bit of lead to keep it flying. But light flies unweariedly and without diminution of speed. When. it has come from the sun in eight minutes, Alpha Cen- tauri in three years, Polaris in forty -five years, other stars in one thousand, its wings are in nowise fatigued, nor is the rapidity of its flight slackened in the least.

It is not the transactions of to-day that we read in the heavens, but it is history, some of it older than the time of Adam. Those stars may have been smitten out of existence decades of centuries ago, but their poured- out light is yet flooding the heavens.

It can go both ways at once in the same place, without interference. We see the light reflected from the new moon to the earth ; reflected back from the house-tops, fields, and waters of earth, to the moon again, and from the moon to us once more — three times in opposite di- rections, in the same place, without interference, and thus we see " the old moon in the arms of the new."

Constitution of Light.

Light was once supposed to be corpuscular, or con- sisting of transmitted particles. It is now known to be the result of undulations in ether. Reference has been made to the minuteness of these undulations. Their velocity is equally wonderful. Put a prism of glass into a ray of light coming into a dark room, and it is

FORCES OF THE SUNBEAM.

25

instantly turned out of its course, some parts more and some less, according to the number of vibrations, and ap- pears as the seven colors on different parts of the screen. Fig. 6 shows the arrangement of colors, and the number of millions of millions of vibrations per second of each.

V, 716 to 765 I, 667 to 699 B, 653 to 658 G, 562 to 610 Y, 510 to 549 O, 478 to 510 R, 396 to 470

Fig. 6.— White Light resolved into Colors.

But the different divisions we call colors are not colors in themselves at all, but simply a different number of vibrations. Color is all in the eye. Violet has in dif- ferent places from 716 to 765,000,000,000,000 of vibra- tions per second ; red has, in different places, from 396 to 470,000,000,000,000 vibrations per second. None of these in any sense are color, but affect the eye different- ly, and we call these different effects color. They are simply various velocities of vibration. An object, like one kind of stripe in our flag, which absorbs all kinds of vibrations except those between 396 and 470,000,000,- 000,000, and reflects those, appears red to us. The field for the stars absorbs and destroys all but those vibra- tions numbering about 653,000,000,000,000 of vibra-

2

26 CREATIVE PROGRESS.

tions per second. A color is a constant creation. Light makes momentary color in the flag. Drake might have written, in the continuous present as well as in the past,

" Freedom mingles with its gorgeous dyes The milky baldrick of the skies, And stripes its pure celestial white With streakings of the morning light."

Every little pansy, tender as fancy, pearled with eva- nescent dew, fresh as a new creation of sunbeams, has power to suppress in one part of its petals all vibrations we call red, in another those we call yellow, and pur- ple, and reflect each of these in other parts of the same tender petal. "Pansies are for thoughts," even more thoughts than poor Ophelia knew. A cloud of smoke that is dense enough to absorb all the faster and weaker vibrations, leaving only the stronger to come through, will show the sun as red ; because the vibrations that produce the impression we have so named are the only ones that have vigor enough to get through. It is like an army charging upon a fortress. Under the deadly fire and fearful obstructions six-sevenths go down, but one-seventh comes through with the glory of victory upon its face.

Light comes in undulations to the eye, as tones of sound to the ear. Must not light also sing? The lowest tone we can hear is made by 16.5 vibrations of air per second ; the highest, so shrill and "fine that nothing lives 'twixt it and silence," is made by 38,000 vibrations per second. Between these extremes lie eleven octaves ; C of the G clef having 25S-J vibrations to the second, and its octave above 517^. Not that sound vibrations cease

FORCES OF THE SUNBEAM. 27

at 38,000, but our organs are not fitted to hear beyond those limitations. If our ears were delicate enough, we could hear even up to the almost infinite vibrations of light. In one of those semi -inspirations we find in Shakspeare's works, he says —

" There's not the smallest orb which thou beholdest, But in his motion like an angel sings, Still quiring to the young-eyed cherubim. Such harmony is in immortal souls; But, whilst this muddy vesture of decay Doth grossly close it in, we cannot hear it."

And that older poetry which is always highest truth says, "The morning stars sing together." We miscon- strued another passage which we could not understand, and did not dare translate as it was written, till science crept up to a perception of the truth that had been stand- ing there for ages, waiting a mind that could take it in. Now we read as it is written — " Thou makest the out-goings of the morning and evening to sing." Were our senses fine enough, we could hear the separate key- note of every individual star. Stars differ in glory and in power, and so in the volume and pitch of their song. Were our hearing sensitive enough, we could hear not only the separate key-notes but the infinite swelling harmony of these myriad stars of the sky, as they pour their mighty tide of united anthems in the ear of God :

" In reason's ear i\\zy all rejoice, And utter forth a glorious voice. Forever singing, as they shine, The hand that made us is divine."

This music is not monotonous. Stars draw near each other, and make a light that i$ unapproachable by mor-

28 CREATIVE PROGRESS.

tals; then the music swells beyond our ability to en- dure. They recede far away, making a light so dim that the music dies away, so near to silence that only spirits can perceive it. No wonder God rejoices in his works. They pour into his ear one ceaseless tide of rapturous song.

Our senses are limited — we have only five, but there is room for many more. Some time we shall be taken out of " this muddy vesture of decay," no longer see the universe through crevices of our prison-house, but shall range through wider fields, explore deeper mys- teries, and discover new worlds, hints of which have never yet been blown across the wide Atlantic that rolls between them and men abiding in the flesh.

Chemistry of Suns revealed by Light.

"When we examine the assemblage of colors spread from the white ray of sunlight, we do not find red sim- ple red, yellow yellow, etc., but there is a vast number of fine microscopic lines of various lengths, parallel — here near together, there far apart, always the same number and the same relative distance, when the same light and prism are used. What new alphabets to new realms of knowledge are these ! Remember, that what we call colors are only various numbers of vibrations of ether. Remember, that every little group in the infinite variety of these vibrations may be affected differently from every other group. One number of these is bent by the prism to where we see what we call the violet, another number to the place we call red. All of the vibrations are destroyed when they strike a surface we call black. A part of them are destroyed when they

FORCES OF THE SUNBEAM. 29

strike a substance we call colored. The rest are reflect- ed, and give the impression of color. In one place on the flag of our nation all vibrations are destroyed ex- cept the red ; in another, all but the blue. Perhaps on that other gorgeous flag, not of our country but of our sun, the flag we call the solar spectrum, all vibrations are destroyed where these dark lines appear. Perhaps this effect is not produced by the surface upon which the rays fall, but by some specific substance in the sun. This is just the truth. Light passing through vapor of sodium has the vibrations that would fall on two nar- row lines in the yellow utterly destroyed, leaving two black spaces. Light passing through vapor of burning iron has some four hundred numbers or kinds of vibra- tions destroyed, leaving that number of black lines; but if the salt or iron be glowing gas, in the source of the light itself the same lines are bright instead of dark.

Thus we have brought to our doors a readable rec- ord of the very substances composing every world hot enough to shine by its own light. Thus, while our flag means all we have of liberty, free as the winds that kiss it, and bright as the stars that shine in it, the flag of the sun means all that it is in constituent elements, all that it is in condition.

We find in our sun many substances known to exist in the earth, and some that we had not discovered when the sun wrote their names, or rather made their mark, in the spectrum. Thus, also, we find that Betelguese and Algol are without any perceivable indications of hydrogen, and Sirius has it in abundance. What a sense of acquaintanceship it gives us to look up and recognize

30 CREATIVE PROGRESS.

the stars whose very substance we know ! If we were transported thither, or beyond, we should not be alto- gether strangers in an unknown realm.

But the stars differ in their constituent elements; every ray that flashes from them bears in its very be- ing proofs of what they are. Hence the eye of Omni- science, seeing a ray of light anywhere in the universe, though gone from its source a thousand years, would be able to tell from what orb it originally came.

Creative Force of Light.

Just above the color vibrations of the nnbraided sun- beam, above the violet, which is the highest number our eyes can detect, is a chemical force ; it works the changes on the glass plate in photography ; it transfig- ures the dark, cold soil into woody fibre, green leaf, downy rose petals, luscious fruit, and far pervasive odor ; it flushes the wide acres of the prairie with grass and flowers, fills the valleys with trees, and covers the hills with corn, a single blade of which all the power of man could not make.

This power is also fit and able to survive. The en- gineer Stephenson once asked Dr. Buckland, " What is the power that drives that train?" pointing to one thundering by. " Well, I suppose it is one of your big engines." " But what drives the engine ?" " Oh, very likely a canny Newcastle driver." " No, sir," said the engineer, " it is sunshine." The doctor was too dull to take it in. Let us see if we can trace such an evident effect to that distant cause. Ages ago the warm sun- shine, falling on the scarcely lifted hills of Pennsylvania, caused the reedy vegetation to grow along the banks of

FORCES OF THE SUNBEAM. 31

shallow seas, accumulated vast amounts of this vegeta- tion, sunk it beneath the sea, roofed it over with sand, compacted the sand into rock, and changed' this vegeta- ble matter — the products of the sunshine — into coal; and when it was ready, lifted it once »more, all garnered for the use of men, roofed over with mighty mountains. AYe mine the coal, bring out the heat, raise the steam, drive the train, so that in the ultimate analyses it is sun- shine that drives the train. These great beds of coal are nothing but condensed sunshine — the sun's great force, through ages gone, preserved for our use to-day. And it is so full of force that a piece of coal that will weigh three pounds (as big as a large pair of fists) has as much power in it as the average man puts into a day's work. Three tons of coal will pump as much water or shovel as much sand as the average man will pump or shovel in a lifetime ; so that if a man proposes to do nothing but work with his muscles, he had better dig three tons of coal and set that to do his work and then die, be- cause his work will be better done, and without any cost for the maintenance of the doer.

Come down below the color vibrations, and we shall find that those which are too infrequent to be visible, manifest as heat. Katurally there will be as many dif- ferent kinds of heat as tints of color, because there is as great a range of numbers of vibration. It is our priv- ilege to sift them apart and sort them over, and find what kinds are best adapted to our various uses.

Take an electric lamp, giving a strong beam of light and heat, and with a plano-convex lens gather it into a single beam and direct it upon a thermometer, twenty feet away, that is made of glass and filled with air. The

32 CREATIVE PROGRESS.

expansion or contraction of this air will indicate the varying amounts of heat. Watch your air-thermometer, on which the beam of heat is pouring, for the result. There is none. And yet there is a strong current of heat there. Put another kind of test of heat beyond it and it appears ; coat the air-thermometer with a bit of black cloth, and that will absorb heat and reveal it. But why not at first ? Because the glass lens stops all the heat that can affect glass. The twenty feet of air absorbs all the heat that affects air, and no kind of heat is left to affect an instrument made of glass and air ; but there are kinds of heat enough to affect instruments made of other things.

A very strong current of heat may be sent right through the heart of a block of ice without melting the ice at all or cooling off the heat in the least. It is done in this way : Send the beam of heat through water in a glass trough, and this absorbs all the heat that can affect water or ice, getting itself hot, and leaving all other kinds of heat to go through the ice beyond ; and appropriate tests show that as much heat comes out on the other side as goes in on this side, and it does not melt the ice at all. Gunpowder may be exploded by heat sent through ice. Dr. Kane, years ago, made this experiment. He was coming down from the north, and fell in with some Esquimaux, whom he was anx- ious to conciliate. He said to the old wizard of the tribe, "I am a wizard; I can bring the sun down out of the heavens with a piece of ice." That was a good deal to say in a country where there was so little sun. " So," he writes, " I took my hatchet, chipped a small piece of ice into the form of a double - convex lens,

FORCES OF THE SUNBEAM. 33

smoothed it with my warm hands, held it up to the sun, and, as the old man was blind, I kindly burned a blister on the back of his hand to show him I could do it."

These are simple illustrations of the various kinds of heat. The best furnace or stove ever invented con- sumes fifteen times as much fuel to produce a given amount of heat as the furnace in our bodies consumes to produce a similar amount. We lay in our supplies of carbon at the breakfast, dinner, and supper table, and keep ourselves warm by economically burning it with the oxygen we breathe.

Heat associated with light has very different quali- ties from that which is not. Sunlight melts ice in the middle, bottom, and top at once. Ice in the spring-time is honey-combed throughout. A piece of ice set in the summer sunshine crumbles into separate crystals. Dark heat only melts the surface.

Nearly all the heat of the sun passes through glass without hinderance ; but take heat from white-hot plat- inum and only seventy-six per cent, of it goes through glass, twenty -four per cent, being so constituted that it cannot pass with facility. Of heat from copper at 752° only six per cent, can go through glass, the other ninety-four per cent, being absorbed by it.

The heat of the sunbeam goes through glass without any hinderance whatever. It streams into the room as freely as if there were no glass there. But what if the furnace or stove heat went through glass with equal facility ? We might as well try to heat our rooms with the window-panes all out, and the blast of winter sweep- ing through them.

The heat of the sun, by its intense vibrations, comes

2*

M CREATIVE PROGRESS.

to the earth dowered with a power which pierces the miles of our atmosphere, but if our air were as pervious to the heat of the earth, this heat would fly away every night, and our temperature would go down to 200° be- low zero. This heat comes with the light, and then, dissociated from it, the number of its vibrations lessen- ed, it is robbed of its power to get away, and remains to work its beneficent ends for our good.

Worlds that are so distant as to receive only -foVff °^ the heat we enjoy, may have atmospheres that retain it all. Indeed it is probable that Mars, that receives but one-quarter as much heat as the earth, has a tempera- ture as high as ours. The poet drew on his imagination when he wrote :

" Who there inhabit must have other powers, Juices, and veins, and sense and life than ours ; One moment's cold like theirs would pierce the bone, Freeze the heart's blood, and turn us all to stone."

The 'power that journeys along the celestial spaces in the flashing sunshine is beyond our comprehension. It accomplishes with ease what man strives in vain to do with all his strength. At West Point there are some links of a chain that was stretched across the river to prevent British ships from ascending; these links were made of two-and-a-quarter-inch iron. A powerful loco- motive might tug in vain at one of them and not stretch it the thousandth part of an inch. But the heat of a single gas-burner, that glows with the preserved sun- light of other ages, when suitably applied to the link, stretches it with ease ; such enormous power has a little heat. There is a certain iron bridge across the Thames at London, resting on arches. The warm sunshine, act-

FORCES OF THE SUNBEAM. 35

ing upon the iron, stations its particles farther and far- ther apart. Since the bottom cannot give way the arches must rise in the middle. As they become long- er they lift the whole bridge, and all the thundering locomotives and miles of goods-trains cannot bring that bridge down again until the power of the sunshine has been withdrawn. There is Bunker Hill Monument, thirty-two feet square at the base, with an elevation of two hundred and twenty feet. The sunshine of every summer's day takes hold of tbat mighty pile of granite with its aerial fingers, lengthens the side affected, and bends the whole great mass as easily as one would bend a whipstock. A few years ago we hung a plummet from the top of this monument to the bottom. At 9 A.M. it began to move toward the west; at noon it swung round toward the north ; in the afternoon it went east of where it first was, and in the night it settled back to its original place.

The sunshine says to the sea, held in the grasp of gravitation, " Rise from your bed ! Let millions of tons of water fly on the wings of the viewless air, hundreds of miles to the distant mountains, and pour there those millions of tons that shall refresh a whole continent, and shall gather in rivers fitted to bear the commerce and the navies of nations." Gravitation says, " I will hold every particle of this ocean as near the centre of the earth as I can." Sunshine speaks with its word of power, and says, " Up and away !" And in the wreath- ing mists of morning these myriads of tons rise in the air, fly away hundreds of miles, and supply all the Ni- agaras, Mississippis and Amazons of earth. The sun says to the earth, wrapped in the mantle of winter,

36 CREATIVE PROGRESS.

" Bloom again ;" and the snows melt, the ice retires, and vegetation breaks forth, birds sing, and spring is about us.

Thus it is evident that every force is constitutional- ly arranged to be overcome by a higher, and all by the highest. Gravitation of earth naturally and legitimate- ly yields to the power of the sun's heat, and then the waters fly into the clouds. It as naturally and legiti- mately yields to the power of mind, and the waters of the Ked Sea are divided and stand " upright as an heap." Water naturally bursts into flame when a bit of potas- sium is thrown into it, and as naturally when Elijah calls the right kind of fire from above. What seems a miracle, and in contravention of law, is only the con- stitutional exercise of higher force over forces organ- ized to be swayed. If law were perfectly rigid, there could be but one force ; but many grades exist from cohesion to mind and spirit. The highest forces are meant to have victory, and thus give the highest order and perfectness.

Across the astronomic spaces reach all these powers, making creation a perpetual process rather than a single act. It almost seems as if light, in its varied capacities, were the embodiment of God's creative power; as if, having said, " Let there be light," he need do nothing else, but allow it to carry forward the creative processes to the end of time. It was Newton, one of the earliest and most acute investigators in this study of light, who said, " I seem to have wandered on the shore of Truth's great ocean, and to have gathered a few pebbles more beautiful than common ; but the vast ocean itself rolls before me undiscovered and unexplored."

EXPERIMENTS.

37

EXPERIMENTS WITH LIGHT.

A light set in a room is seen from every place ; hence light streams in every possible direction. If put in the centre of a hollow sphere, every point of the surface will be equally illumined. If put in a sphere of twice the diameter, the same light will fall on all the larger surface. The surfaces of spheres are as the squares of their diameters ; hence, in the larger sphere the surface is illumined only one -quarter as much as the smaller. The same is true of large and small rooms. In Fig. 7 it is ap-

Fig. 7.

parent that the light that falls on the first square is spread, at twice the distance, over the second square, which is four times as large, and at three times the distance over nine times the surface. The van-ing amount of light received by each planet is also shown in fractions above each world, the amount received by the earth being 1.

Fig. 8.— Measuring Intensities of Lights.

The intensity of light is easily measured. Let two lights of different brightness, as in Fig. 8, cast shadows on the same screen. Arrange them as to distance so that both shadows shall be equally dark. Let them fall side by side, and study them carefully. Measure the respective distances. Suppose one is twenty inches, the other forty. Light varies as the square

38 CREATIVE PROGRESS.

of the distance : the square of 20 is 400, of 40 is 1600. Divide 1600 by 400, and the result is that one light is four times as bright as the other.

Light can be handled, directed, and bent, as well as iron bars. Darken a room and admit a beam of sunlight through a shutter, or a ray of lamp- light through the key-hole. If there is dust in the room it will be observed that light goes in straight lines. Because of this men are able to arrange houses and trees in rows, the hunter aims his rifle correctly, and the as- tronomer projects straight lines to infinity. Take a hand-mirror, or bet-

Fig. 9 Reflection and Diffusion of Ligh

ter, a piece of glass coated on one side with black varnish, and you can send your ray anywhere. By using two mirrors, or having an assistant and using several, you can cause a ray of light to turn as many corners as you please. I once saw Mr. Tyndall send a ray into a glass jar filled with smoke (Fig. 9). Admitting a slender ray through a small hole in a card over the mouth, one ray appeared ; removing the cover, the whole jar was luminous ; as the smoke disappeared in spots cavities of dark- ness appeared. Turn the same ray into a tumbler of water, it becomes

EXPERIMENTS.

39

faintly visible ; stir into it a teaspoonful of milk, then turn in the ray of sunlight, and it glows like a lamp, illuminating the whole room. These experiments show how the straight rays of the sun are diffused in every direction over the earth.

Set a small light near one edge of a mirror ; then, by putting the eye near the opposite edge, you see almost as many flames as you please from the multiplied reflections. How can this be accounted for ?

Into your beam of sunlight, admitted through a half-inch hole, put the mirror at an oblique angle ; you can arrange it so as to throw half a dozen bright spots on the opposite wall.

In Fig. 10 the sunbeam enters at A, and, striking the mirror m at a, is partly reflected to 1 on the wall, and partly enters the glass, passes

Fig. 10.— Mauifold Reflections.

through to the silvered back at B, and is totally reflected to 6, where it again divides, some of it going to the wall at 2, and the rest, continuing to make the same reflections and divisions, causes spots 3, 4, 5, etc. The brightest spot is at No. 2, because the silvered glass at B is the best re- flector and has the most light.

When the discovery of the moons of Mars was announced in 1877, it was also widely published that they could be seen by a mirror. Of course this is impossible. The point of light mistaken for the moon in this sec- ondary reflection was caused by holding the mirror in an oblique position.

Take a small piece of mirror, say an inch in surface, and putting under it three little pellets of wax, putty, or clay, set it on the wrist, with one of the pellets on the pulse. Hold the mirror steadily in the beam of ligM, and the frequency and prominence of each pulse-beat will be indicated by the tossing spot of light on the Avail. If the operator becomes excited the fact will be evident to all observers.

Place a coin in a basin (Fig. 11), and set it so that the rim will con- ceal the coin from the eye. Pour in water, and the coin will appear

CREATIVE PROGRESS.

Fig. 11.

to rise into sight. When light passes from a medium of one density to a medium of another, its direction is changed. Thus a stick in water seems bent. Ships below the horizon are sometimes seen above, because of the

different density of the layers of air.

Thus light coming from the interstellar spaces, and entering our atmos- phere, is bent down more and more by its increasing density. The effect is greatest when the sun or star is near the horizon, none at all in the zenith. This brings the object into view before it is risen. Allowance for this displacement is made in all delicate astronomical observations.

Fig. 12. — Atmospherical Refraction.

Notice on the floor the shadow of the window-frames. The glass of almost every window is so bent as to turn the sunlight aside enough to obliterate some of the shadows or increase their thickness.

DECOMPOSITION OF LIGHT.

Admit the sunbeam through a slit one inch long and one-twentieth of an inch wide. Pass it through a prism. Either purchase one or make it of three plain pieces of glass one and a half inch wide by six inches long, fastened together in triangular shape — fasten the edges with hot wax and fill it with water ; then on a screen or wall you will have the colors of the rainbow, not merely seven but seventy, if your eyes are sharp enough.

Take a bit of red paper that matches the red color of the spectrum. Move it along the line of colors toward the violet. In the orange it is dark, in the yellow darker, in the green and all beyond, black. That is because there are no more red rays to be reflected by it. So a green ob- ject is true to its color only in the green rays, and black elsewhere. All these colors may be recombined by a second prism into white light.

III.

ASTRONOMICAL INSTRUMENTS.

The eyes of the Lord are in every place." — Proverbs xv.

"Man, having one kind of an eye given him by his Maker, proceeds to construct two other kinds. He makes one that magnifies invisible objects thousands of times, so that a dull razor-edge appears as thick as three fingers, until the amazing beauty of color and form in infinitesimal ob- jects is entrancingly apparent, and he knows that God's care of least things is infinite. Then he makes the other kind four or six feet in di- ameter, and penetrates the immensities of space thousands of tines be- yond where his natural eye can pierce, until he sees that God s immensi- ties of worlds are infinite also." — BISHOP FOSTER.

THE TELESCOPE. 43

III. THE TELESCOPE.

FREQFEXT allusion has been made in the previous chapter to discovered results. It is necessary to under- stand more clearly the process by which such results have been obtained. Some astronomical instruments are of the simplest character, some most delicate and complex. "Vyhen a man smokes a piece of glass, in or- der to see an eclipse of the sun, he makes a simple instrument. Ferguson, lying on his back and slipping beads on a string at a certain distance above his eye, measured the relative distances of the stars. The use of more complex instruments commenced when Galileo applied the telescope to the heavens. He cannot be said to have invented the telescope, but he certainly con- structed his own without a pattern, and used it to good purpose. It consists of a lens, O B (Fig. 13), which

^

B

Fig. 13.— Refracting Telescope.

acts as a multiple prism to bend all the rays to one point at R. Place the eye there, and it receives as much light as if it were as large as the lens O B. The rays, however, are convergent, and the point difficult to

44 ASTRONOMICAL INSTRUMENTS.

find. Hence there is placed at R a concave lens, pass- ing through which the rays emerge in parallel lines, and are received by the eye. Opera-glasses are made upon precisely this principle to-day, because they can be made conveniently short.

If, instead of a concave lens at R, converting the con- verging rays into parallel ones, we place a convex or magnifying lens, the minute image is enlarged as much as an object seems diminished when the telescope is reversed. This is the grand principle of the refracting telescope. Difficulties innumerable arise as we attempt to enlarge the instruments. These have been over- come, one after another, until it is now felt that the best modern telescope, with an object lens of twenty-six inches, has fully reached the limit of optical power.

The Reflecting Telescope.

This is the only kind of instrument differing radi- cally from the refracting one already described. It re- ceives the light in a concave mirror, M (Fig. 14), which

Fig. 14.— Reflecting Telescope.

reflects it to the focus F, producing the same result as the lens of the refracting telescope. At B a mirror may be placed obliquely, reflecting the image at right angles to the eye, outside the tube, in which case it is called the Newtonian telescope ; or a mirror at R may be placed perpendicularly, and send the rays through

THE REFLECT IX <r TELESCOPE. 45

an opening in the mirror at M. This form is called the Gregorian telescope. Or the mirror M may be slightly inclined to the coining rays, so as to bring the point F entirely outside the tube, in which case it is called the Herschelian telescope. In either case the image may be magnified, as in the refracting telescope.

Reflecting telescopes are made of all sizes, up to the Cyclopean eye of the one constructed by Lord Rosse, which is six feet in diameter. The form of instru- ment to be preferred depends on the use to which it is to be put. The loss of light in passing through glass lenses is about two -tenths. The loss by reflection is often one-half. In view of this peculiarity and many others, it is held that a twenty-six-inch refractor is fully equal to any six-foot reflector.

The mounting of large telescopes demands the high- est engineering ability. The whole instrument, with its vast weight of a twenty-six-inch glass lens, with its accompanying tube and appurtenances, must be pointed as nicely as a rifle, and held as steadily as the axis of the globe. To give it the required steadiness, the foun- dation on which it is placed is sunk deep in the earth, far from rail or other roads, and no part of the observ- atory is allowed to touch this support. When a star is once found, the earth swiftly rotates the telescope away from it, and it passes out of the field. To avoid this, clock-work is so arranged that the great telescope fol- lows the star by the hour, if required. It will take a star at its eastern rising, and hold it constantly in view while it climbs to the meridian and sinks in the west iFig. 15). The reflector demands still more difficult en- gineering. That of Lord Rosse has a metallic mirror

4:6

ASTRONOMICAL INSTRUMENTS.

Fig. 15, — Cambridge Equatorial.

weighing six tons, a tube fifty-six feet long, which, with its appurtenances, weighs seven tons more. It moves between two walls only 20° east and west. The new Paris reflector (Fig. 16) has a much wider range of movement

The Spectroscope.

A spectrum is a collection of the colors which are dispersed by a prism from any given light. If it is sun- light, it is a solar spectrum; if the source of light is a

Fiji. 16.— New Paris Reflector.

THE SPECTROSCOPE. 49

star, candle, glowing metal, or gas, it is the spectrum of a star, candle, glowing metal, or gas. An instrument to see these spectra is called a spectroscope. Consider- ing the infinite variety of light, and its easy modifica- tion and absorption, we should expect an immense number of spectra. A mere prism disperses the light so imperfectly that different orders of vibrations, per- ceived as colors, are mingled. No eye can tell where one commences or ends. Such a spectrum is said to be impure. What we want is that each point in the spectrum should be made of rays of the same number of vibrations. As we can let only a small beam of light pass through the prism, in studying celestial objects with a telescope and spectroscope we must, in every instance, contract the aperture of the instrument un- til we get only a small beam of light. In or- der to have the colors thoroughly dispersed, the best instruments pass the beam of light through a series of prisms called a bat- tery, each one spread- ing farther the colors which the previous ones had spread. In Fig. 17 the ray is seen

. . . Fig. 17 — Spectroscope, with Battery of Prisms.

entering through the

telescope A, which renders the rays parallel, and pass- ing through the prisms out to telescope B, where the

3

50

ASTR GNOMICAL IN8TR UMENTS.

spectrum can be examined on the retina of the eye for a screen. In order to still farther disperse the rays, some batteries receive the ray from the last prism at 0 upon an oblique mirror, send it up a little to another, which delivers it again to the prism to make its journey back again through them all, and come out to be ex- amined just above where it entered the first prism.

Attached to the examining telescope is a diamond- ruled scale of glass, enabling us to fix the position of any line with great exactness.

In Fig. 18 is seen, in the lower part, a spectrum of the sun, with about a score of its thousands of lines

D

II h O F c ft E DCBA

Fig. IS.— Spectra of glowing Hydrogen nnd the Sun.

made evident. In the upper part is seen the spectrum of bright lines given by glowing hydrogen gas. These lines are given by no other known gas; they are its autograph. It is readily observed that they precisely correspond with certain dark lines in the solar spec- trum. Hence we easily know that a glowing gas gives the same bright lines that it absorbs from the light of another source passing through it — that is, glowing gas gives out the same rays of light that it absorbs when it is not glowing.

The subject becomes clearer by a study of the chro- molithic plate. No. 1 represents the solar spectrum, with a few of its lines on an accurately graduated scale.

THE SPECTROSCOPE. 51

No. 3 shows the bright line of glowing sodium, and, corresponding to a dark line in the solar spectrum, shows the presence of salt in that body. No. 2 shows that potassium has some violet rays, but not all ; and there being, no dark line to correspond in the solar spectrum, we infer its absence from the sun. No. 6 shows the numerous lines and bands of barium — several red, orange, yellow, and four are very bright green ones. The lines given by any volatilized substances are al- ways in the same place on the scale.

A patient study of these signs of substances reveals richer results than a study of the coniform characters engraved on Assyrian slabs; for one is the handwri- ting of men, the other the handwriting of God.

One of the most difficult and delicate problems solved by the spectroscope is the approach or departure of a light-giving body in the line of sight. Stand before a locomotive a mile away, you cannot tell whether it ap- proaches or recedes, yet it will dash by in a minute. How can the movements of the stars be comprehended when they are at such an immeasurable distance?

It can best be illustrated by music. The note C of the G clef is made by two hundred and fifty-seven vi- brations of air per second. Twice as many vibrations per second would give us the note C an octave above. Sound travels at the rate of three hundred and sixty- four yards per second. If the source of these two hun- dred and fifty -seven vibrations could approach us at three hundred and sixty-four yards per second, it is ob- vious that twice as many waves would be put into a given space, and we should hear the upper C when only waves enough were made for the lower C. The same

52 ASTRONOMICAL INSTRUMENTS.

result would appear if we carried our ear toward the sound fast enough to take up twice as many waves as though we stood still. This is apparent to every ob- server in a railway train. The whistle of an approach- ing locomotive gives one tone; it passes, and we in- stantly detect another. Let two trains, running at a speed of thirty-six yards a second, approach each oth- er. Let the whistle of one sound the note E, three hundred and twenty -three vibrations per second. It will be heard on the other as the note G, three hun- dred and eighty- eight vibrations per second; for the speed of each train crowds the vibrations into one-tenth less room, adding 32 + vibrations per second, making three hundred and eighty-eight in all. The trains pass. The vibrations are put into one -tenth more space by the whistle making them, and the other train allows only nine-tenths of what there are to overtake the ear. Each subtracts 32+ vibrations from three hundred and twenty-three, leaving only two hundred and fifty-eight, which is the note C. Yet the note E was constantly uttered.

If a source of light approach or depart, it will have a similar effect on the light waves. How shall we detect it ? If a star approach us, it puts a greater number of waves into an inch, and shortens their length. If it re- cedes, it increases the length of the wave — puts a less number into an inch. If a body giving only the num- ber of vibrations we call green were to approach suf- ficiently fast, it would crowd in vibrations enough to appear what we call blue, indigo, or even violet, accord- ing to its speed. If it receded sufficiently fast, it would leave behind it only vibrations enough to fill up the

1. Solar Spectrum.

2. Spectrum of Potassium

3. Spectrum, « 4 Spectrum o

Harper fc-Brotli

of Sodium of Strontium..

.tiers .Kew York,

5. Spectrum of Calcium

6. Spectrum, of Barium

THE SPECTROSCOPE. 53

space with what we call yellow, orange, or red, accord- ing to its speed ; yet it would be green, and green only, all the time. But how detect the change? If red waves are shortened they become orange in color; and from below the red other rays, too far apart to be seen by the eye, being shortened, become visible as red, and we can- not know that anything has taken place. So, if a star recedes fast enough, violet vibrations being lengthened become indigo ; and from above the violet other rays, too short to be seen, become lengthened into visible vio- let, and we can detect no movement of the colors. The dark lines of the spectrum are the cutting out of rays of definite wave-lengths. If the color spectrum moves away, they move with it, and away from their proper place in the ordinary spectrum. If, then, we find them toward the red end, the star is receding ; if toward the violet end, it is approaching. Turn the instrument on the centre of the sun. The dark lines take their appro- priate place, and are recognized on the ruled scale. Turn it on one edge, that is approaching us one and a quarter miles a second by the revolution of the sun on its axis, the spectral lines move toward the violet end ; turn the spectroscope toward the other edge of the sun, it is re- ceding from us one and a quarter miles a second by reason of the axial revolution, and the spectral lines move toward the red end. Turn it near the spots, and it reveals the mighty up -rush in one place and the down-rush in another of one hundred miles a second. We speak of it as an easy matter, but it is a problem of the greatest delicacy, almost defying the mind of man to read the movements of matter.

It should be recognized that Professor Young, of

54 ASTRONOMICAL INSTRUMENTS.

Princeton, is the most successful operator in this recent realm of science. He already proposes to correct the former estimate of the sun's axial revolutions, derived from observing its spots, by the surer process of observ- ing accelerated and retarded light.

Within a very few years this wonderful instrument, the spectroscope, has made amazing discoveries. In chemistry it reveals substances never known before ; in analysis it is delicate to the detection of the mill- ionth of a grain. It is the most deft handmaid of chemistry, the arts, of medical science, and astronomy. It tells the chemical constitution of the sun, the move- ments taking place, the nature of comets, and nebulae. By the spectroscope we know that the atmospheres of Venus and Mars are .like our own ; that those of Jupi- ter and Saturn are very unlike ; it tells us which stars approach and which recede, and just how one star dif- fereth from another in glory and substance.

In the near future we shall have the brilliant and diversely colored flowers of the sky as well classified into orders and species as are the flowers of the earth.

CELESTIAL MEASUREMENTS.

"Who hath measured the waters in the hollow of his hand, and meted out heaven with the span ? Mine hand also hath laid the foundation of the earth, and my rigni hand hath spanned the heavens." — Isa. xl. 12; xlviii. 13.

6* Go to yon tower, where busy science plies Her vast antennae, feeling thro1 the skies ; That little vernier, on whose slender lines The midnight taper tremhles as it shines, A silent index, tracks the planets' inarch In all their wanderings thro' the ethereal arch, Tells through the mist whd'd cazzled Mercury burns, And marks the spot where Uranus returns.

" So, till by wrong or negligence effaced, The living index which thy Maker traced Kepeats the line each starry virtue draws Through the wide circuit of creation's laws : Still tracks unchanged the everlasting ray Where the dark shadows of temptation stray ; But, once defaced, forgets the orbs of light, And leaves thee wandering o'er the expanse of night."

OLIVER WENDELL HOLMES.

USES OF ASTRONOMY. 57

IY.

CELESTIAL MEASUREMENTS.

WE know that astronomy lias what are called practi- cal uses. If a ship had been driven by Euroclydon ten times fourteen days and nights without sun or star ap- pearing, a moment's glance into the clear sky from the heaving deck, by a very slightly educated sailor, would tell within one hundred rods where he was, and de- termine the distance and way to the nearest port. We know that, in all final and exact surveying, positions must be fixed by the stars. Earth's landmarks are un- certain and easily removed ; those which we get from, the heavens are stable and exact.

In 1878 the United States steam -ship Enterprise was sent to survey the Amazon. Every night a " star party" went ashore to fix the exact latitude and lon- gitude by observations of the stars. Our real land- marks are not the pillars we rear, but the stars millions of miles away. All our standards of time are taken from the stars ; every railway train runs by their time to avoid collision ; by them all factories start and stop. Indeed, we are ruled by the stars even more than the old astrologers imagined.

Man's finest mechanism, highest thought, and broad- est exercise of the creative faculty have been inspired by astronomy. No other instruments approximate in delicacy those which explore the heavens; no other

58 CELESTIAL MEASUREMENTS.

system of thought can draw such vast and certain con- clusions from its premises. " Too low they build who build beneath the stars ;" we should lay our foundations in the skies, and then build upward.

We have been placed on the outside of this earth, in- stead of the inside, in order that we may look abroad. We are carried about, through unappreciable distance, at the inconceivable velocity of one thousand miles a minute, to give us different points of vision. The earth, on its softly-spinning axle, never jars enough to unnest a bird or wake a child ; hence the foundations of our observatories are firm, and our measurements ex- act. Whoever studies astronomy, under proper guid- ance and in the right spirit, grows in thought and feel- ing, and becomes more appreciative of the Creator.

Celestial Movements.

Let it not be supposed that a mastery of mathematics and a finished education are necessary to understand the results of astronomical research. It took at first the highest power of mind to make the discoveries that are now laid at the feet of the lowliest. It took sublime faith, courage, and the results of ages of experience in navigation, to enable Columbus to discover that path to the New World which now any little boat can follow. Ages of experience and genius are stored up in a loco- motive, but quite an unlettered man can drive it. It is the work of genius to render difficult matters plain, abstruse thoughts clear.

A brief explanation of a few terms will make the prin- ciples of world inspection easily understood. Imagine a perfect circle thirty feet in diameter — that is, create

USES OF ASTRONOMY. • 59

one (Fig. 19). Draw through it a diameter horizontally, another perpendicularly. The angles made by the intersecting lines are each said to be ninety degrees, marked thus °. The arc of a circle included between any two of the lines is also 90°. Every cir- cle, great or small, is divided into these 360°. If the sun rose in the east and came to the zenith at noon, it would have passed 90°. When it set in the west it would have traversed half the circle, or 180°. In Fig. 20 the angle of the lines meas-

Fig. 20.— Illustration of Angles.

ured on the graduated arc is 10°. The mountain is 10° high, the world 10° in diameter, the comet moves 10° a day, the stars are 10° apart. The height of the moun- tain, the diameter of the world, the velocity of the comet, and the distance between the stars, depend on the distance of each from the point of sight. Every degree is divided into 60 minutes (marked '), and every minute into 60 seconds (marked ").

Imagine yourself inside a perfect sphere one hundred feet in diameter, with the interior surface above, around, and below studded with fixed bright points like stars. The familiar constellations of night might be blazoned there in due proportion.

If this star-sprent sphere were made to revolve once in twenty-four hours, all the stars would successively

60 . CELESTIAL MEASUREMENTS.

pass in review. How easily we could measure distances between stars, from a certain fixed meridian, or the equator! How easily we could tell when any particular star would culminate ! It is as easy to take all these measurements when our earthly observatory is steadi- ly revolved within the sphere of circumambient stars. Stars can be mapped as readily as the streets of a great city. Looking down on it in the night, one could trace the lines of lighted streets, and judge something of its extent and regularity. But the few lamps of even- ing would suggest little of the greatness of the public buildings, the magnificent enterprise and commerce of its citizens, or the intelligence of its scholars. Looking up to the lamps of the celestial city, one can judge something of its extent and regularity ; but they sug- gest little of the magnificence of the many mansions.

Stars are reckoned as so many degrees, minutes, and seconds from each other, from the zenith, or from a giv- en meridian, or from the equator. Thus the stars called the Pointers, in the Great Bear, are 5° apart ; the near- est one is 29° from the Pole Star, which is some 40° above the horizon at Philadelphia. In going to Eng- land you creep up toward the north end of the earth, till the Pole Star is 54° high. It stays near its place among the stars continually,

"Of whose true-fixed and resting quality There is no fellow in the firmament."

How to Measure.

Suppose a telescope, fixed to a mural circle, to revolve on an axis, as in Fig. 21 ; point it horizontally at a star ;

USES OF ASTRONOMY.

61

turn it up perpendicular to another star. Of course the two stars are 90° apart, and the graduated scale, which is attached to the outer edge of the circle, shows a revolution of a quarter circle, or 90°. But a perfect accuracy of measurement must be sought ; for to mis- take the breadth of a hair, seen at the distance of one hundred and twenty-five feet, would cause an error of 3,000,000 miles at the distance of the sun, and iru-

Fig. 521.— Mural Circle.

mensely more at the distance of the stars. The cor- rection of an inaccuracy of no greater magnitude than that has reduced our estimate of the distance of our sun 3,000,000 miles.

Consider the nicety of the work. Suppose the grad- uated scale to be thirty feet in circumference. Divided into 360°, each would be one inch long. Divide each degree into 60', each one is ^ of an inch long. It takes good eyesight to discern it. But each minute must be

62 CELESTIAL MEASUREMENTS.

divided into 60", and these must not only be noted, but even tenths and Irundredths of seconds must be discern- ed. Of course they are not seen by the naked eye; some mechanical contrivance must be called in to assist. A watch loses two minutes a week, and hence is unrelia- ble. It is taken to a watch-maker that every single sec- ond may be quickened -rnrliru Pai't of itself. Now YTroinr part of a second would be a small interval of time to measure, but it must be under control. ' If the tempera- ture of a summer morning rises ten or twenty degrees we scarcely notice it ; but the magnetic tasimeter meas- ures -soVo- of' a degree.

Come to earthly matters. In 1874, after nearly twenty -eight years' work, the State of Massachusetts opened a tunnel nearly five miles long through the Iloosac Mountains. In the early part of the work the engineers sunk a shaft near the middle 1028 feet deep. Then the question to be settled was where to go so as to meet the approaching excavations from the east and west. A compass could not be relied on under a moun- tain. The line must be mechanically fixed. A little divergence at the starting-point would become so great, miles away, that the excavations might pass each other without meeting; the grade must also rise toward the central shaft, and fall in working away from it ; but the lines were fixed with such infinitesimal accuracy that, when the one going west from the eastern portal and the one going east from the shaft met in the heart of the mountain, the western line was only one-eighth of an inch too high, and three-sixteenths of an inch too far north. To reach this perfect result they had to trian- gulate from the eastern portal to distant mountain

USES OF ASTRONOMY.

63

peaks, and thence down the valley to the central shaft, and thus fix the direction of the proposed line across the mouth of the shaft. Plumb-lines were then drop- ped one thousand and twenty-eight feet, and thus the line at the bottom was fixed.

Three attempts were made— in 1867, 1870, and 1872 — to fix the exact time-distance between Greenwich and Washington. These three separate efforts do not differ one-tenth of a second. Such demonstrable results on earth greatly increase our confidence in similar measure- ments in the skies.

A scale is frequently affixed to a pocket -rule, by which we can easily measure one-hundredth of an inch (Fig. 22). The upper and lower line is divided into tenths of an inch. Ob- serve the slanting line at the right hand. It leans from the perpendicular one- tenth of an inch, as shown by noticing where it reaches the top line. When it reaches the second horizontal line it has left the perpendicular one-tenth of that tenth — that is, one-hundredth. The intersection marks -£?& of an inch from one end, and one-hundredth from the other.

When division-lines, on measures of great nicety, get too fine to be read by the eye, we use the microscope. By its means we are able to count 112,000 lines ruled on a glass plate within an inch. The smallest object that can be seen by a keen eye makes an angle of 40", but by putting six microscopes on the scale of the tel- escope on the mural circle, we are able to reach an exactness of 0".l, or -g-g-Vo of an inch. This instru- ment is used to measure the declination of stars, or an-

Fig. 22.

G4 CELESTIAL MEASUREMENTS.

gular distance north or south of the equator. Thus a star's place in two directions is exactly fixed. "When the telescope is mounted on two pillars instead of the face of a wall, it is called a transit instrument. This is used to determine the time of transit of a star over the meridian, and if the transit instrument is provided with a graduated circle it can also be used for the same purposes as the mural circle. Man's capacity to meas- ure exactly is indicated in his ascertainment of the length of waves of light. It is easy to measure the three hundred feet distance between the crests of storm-waves in the wide Atlantic ; easy to measure the different wave-lengths of the different tones of musical sounds. So men measure the lengths of the undula- tions of light. The shortest is of the violet light, 154.84 ten-millionths of an inch. By the horizontal pendulum Professor Eoot has made rainnnnnF °^ an mc^ aPParent- The next elements of accuracy must be perfect time and perfect notation of time. As has been said, we get our time from the stars. Thus the infinite and heavenly dominates the finite and earthly. Clocks are set to the invariable sidereal time. Sidereal noon is when we have turned ourselves under the point where the sun crosses the equator in March, called the vernal equinox. Sidereal clocks are figured to indicate twenty- four hours in a day : they tick exact seconds. To map stars we wish to know the exact second when they cross the meridian, or the north and south line in the celestial dome above us. The telescope (Fig. 21, p. 61) swings ex- actly north and south. In its focus a set of fine threads of spider-lines is placed (Fig. 23). The telescope is set just high enough, so that by the rolling over of the earth

USES OF ASTRONOMY.

65

Fig. -23.— Transit of a Star noted.

the star will come into the field jnst above the horizon- tal thread. The observer notes the exact second and tenth of a second when the star reaches each vertical thread in the instrument, adds together the times and divides by five to get the average, and the exact time is reached.

But man is not reliable enough to observe and record with sufficient accuracy. Some, in their excitement, an- ticipate its positive passage, and some cannot get their slow mental machinery in motion till after it has made the transit. Moreover, men fall into a habit of esti- mating some numbers of tenths of a second oftener than others. It will be found that a given observer will say three tenths or seven tenths oftener than four or eight. He is falling into ruts, and not trustworthy. General O. M. Mitchel. who had been director of the Cincinnati Observatory, once told one of his staff -offi- cers that he was late at an appointment. " Only a few minutes," said the officer, apologetically. "Sir," said the general, "where I have been accustomed to work, hundredths of a second are too important to be neglected." And it is to the rare genius of this astron- omer, and to others, that we owe the mechanical accu- racy that we now attain. The clock is made to mark its seconds on paper wrapped around a revolving cylinder. Under the observer's fingers is an electric key. This he can touch at the instant of the transit of the star

66 CELESTIAL MEASUREMENTS.

over eacli wire, and thus put his observation on the same line between the seconds dotted by the clock. Of course these distances can be measured to minute fractional parts of a second.

But it has been found that it takes an appreciable time for every observer to get a thing into his head and out of his finger-ends, and it takes some observers longer than others. A dozen rnen, seeing an electric spark, are liable to bring down their recording marks in a dozen different places on the revolving paper. Hence the time that it takes for each man to get a thing into his head and out of his fingers is ascertained. This time is called his personal equation, and is subtracted from all of his observations in order to get at the true time; so willing are men to be exact about material matters. Can it be thought that moral and spiritual matters have no precision? Thus distances east or west from any given star or meridian are secured ; those north and south from the equator or the zenith are as easily fixed, and thus we make such accurate maps of the heavens that any movements in the far-off stars — so far that it may take centuries to render the swiftest movements appreciable — may at length be recognized and account- ed for.

We now come to a little study of the modes of measuring distances. Create a perfect square (Fig. 24); draw a diagonal line. The square angles are 90°, the divided angles give two of 45° each. Now the base A B is equal to the perpendicu-

lar A C. Now any point — C, where a F\S. 24.

perpendicular, A C, and a diagonal, B C, meet — will be

USES OF ASTRONOMY.

67

as far from A as B is. It makes no difference if a river

flows between A and C, and we cannot go over it; we

can measure its distance as easily as

if we could. Set a table four feet by ff/?

eight out-doors (Fig. 25) ; so arrange it /•

that, looking along one end, the line / «

of sight just strikes a tree the other / »

side of the river. Go to the other

end, and, looking toward the tree, you

find the line of sight to the tree falls

an inch from the end of the table on

the farther side. The lines, therefore,

approach each other one inch in every

four feet, and will come together at a

tree three hundred and eighty -four ._jii

feet away.

The next process is to measure the Fig. 25.— Measuring Dis- height or magnitude of objects at an ascertained distance. Put two pins in a stick half an inch apart (Fig. 26). Hold it up two feet from the eye,

Fig. 26.— Measuring Elevations.

and let the upper pin fall in line with your eye and the top of a distant church steeple, and the lower pin in line with the bottom of the church and your eye. If the church is three-fourths of a mile away, it must be eighty-two feet high ; if a mile away, it must be one hundred and ten feet high. For if two lines spread

68 CELESTIAL MEASUREMENTS.

one-half an inch going two feet, in going four feet they will spread an inch, and in going a mile, or five thou- sand two hundred and eighty feet, they will spread out one -fourth as many inches, viz., thirteen hundred and twenty — that is, one hundred and ten feet. Of course these are not exact methods of measurement, and would not be correct to a hair at one hundred and twenty-five feet, but they perfectly illustrate the true methods of measurement.

Imagine a base line ten inches long. At each end erect a perpendicular line. If they are carried to in- finity they will never meet : will be forever ten inches apart. But at the distance of a foot from the base line incline one line toward the other TiminnroTr of an inch, and the lines will come together at a distance of three hundred miles. That new angle differs from the for- mer right angle almost intinitesimally, but it may be measured. Its value is about three-tenths of a second. If we lengthen the base line from ten inches to all the miles we can command, of course the point of meet- ing will be proportionally more distant. The angle made by the lines where they come together will be obviously the same as the angle of divergence from a right angle at this end. That angle is called the parallax of any body, and is the angle that would be made by two lines coming from that body to the two ends of any conventional base, as the semi-diameter of the earth. That that angle would vary according to the various dis- tances is easily seen by Fig. 27.

Let O P be the base. This would subtend a greater angle seen from star A than from star E. Let B be far enough away, and O P would become invisible, and B

USES OF ASTRONOMY. 69

would have no parallax for that base. Thus the moon has a parallax of 57' with the semi-equatorial diameter of the earth for a base. And the sun has a parallax 8".85 on the same base. It is not necessary to confine ourselves to right angles in these measurements, for the same principles hold true in any angles. Now, suppose two observers on the equator should look at the moon at the same instant. One is on the top of Cotopaxi, on the west coast of South America, and one on the west coast of Africa. They are 90° apart — half the earth's diam- eter between them. The one on Cotopaxi sees it exactly overhead, at an angle of 90° with the earth's diameter. The one on the coast of Africa sees its angle with the same line to be 89° 3'— that is, its parallax is 57'. Try the same

experiment on the sun farther away, as is seen in Fig. 27, and its smaller parallax is found to be only 8".85.

It is not necessary for two observers to actually sta- tion themselves at two distant parts of the earth in order to determine a parallax. If an observer could go from one end of the base-line to the other, he could determine both angles. Every observer is actually car- ried along through space by two motions : one is that of the earth's revolution of one thousand miles an hour around the axis ; and the other is the movement of the earth around the sun of one thousand miles in a minute. Hence we can have the diameter not only of

70 CELESTIAL MEASUREMENTS.

the earth (eight thousand miles) for a base-line, but the diameter of the earth's orbit (184,000,000 miles), or any part of it, for such a base. Two observers at the ends of the earth's diameter, looking at a star at the same in- stant, would find that it made the same angle at both ends ; it has no parallax on so short a base. We must seek a longer one. Observe a certain star on the 21st

o

of March ; then let us traverse the realms of space for six months, at one thousand miles a minute. We come round in our orbit to a point opposite where we were six months ago, with 184,000,000 of miles between the points. Now, with this for a base-line, measure the an- gles of the same stars : it is the same angle. Sitting in my study here, I glance out of the window and dis- cern separate bricks, in houses five hundred feet away, with my unaided eye ; they subtend a discernible an- gle. But one thousand feet away I cannot distinguish individual bricks ; their width, being only two inches, does not subtend an angle apprehensible to my vision. So at these distant stars the earth's enormous orbit, if lying like a blazing ring in space, with the world set on its edge like a pearl, and the sun blazing like a diamond in the centre, would all shrink to a mere point. ]S"ot quite to a point from the nearest stars, or we should never be able to measure the distance of any of them. Professor Airy says that our orbit, seen from the nearest star, would be the same as a circle six-tenths of an inch in diameter seen at the distance of a mile: it would all be hidden by a thread one-twenty-fifth of an inch in diameter, held six hundred and fifty feet from the eye. If a straight line could be drawn from a star, Sirius in the east to the star Vega in the west, touching our

USES OF ASTRONOMY. 71

earth's orbit on one side, as T K A (Fig. 28), and a line were to be drawn six months later from

same stars, touching our B ~~-^

earth's orbit on the oth- ^"^"•^n

erside,asKBT,sucha Fi--28-

line would not diverge sufficiently from a straight line for us to detect its divergence. ^Numerous vain at- tempts had been made, up to the year 1835, to detect and measure the angle of parallax by which we could rescue some one or more of the stars from the inconceiv- able depths of space, and ascertain their distance from us. .We are ever impelled to triumph over what is de- clared to be unconquerable. There are peaks in the Alps no man has ever climbed. They are assaulted every year by men zealous of more worlds to conquer. So these greater heights of the heavens have been as- saulted, till some ambitious spirits have outsoared even imagination by the certainties of mathematics.

It is obvious that if one star were three times as far from us as another, the nearer one would seem to be displaced by our movement in our orbit three times as much as the other ; so, by comparing one star with an- other, we reach a ground of judgment. The ascertain- ment of longitude at sea by means of the moon affords a good illustration. Along the track where the moon sails, nine bright stars, four planets, and the sun have been selected. The nautical almanacs give the distance of the moon from these successive stars every hour in the night for three years in advance. The sailor can measure the distance at any time by his sextant. Looking from the world at D (Fig. 29), the distance of the moon and

72 CELESTIAL MEASUREMENTS.

star is A E, which is given in the almanac. Looking from C, the distance is only B E, which enables even the uneducated sailor to find the distance, C D, on the earth, or his distance from Greenwich.

Fig. 29.— Mode of Ascertaining Longitude.

So, by comparisons of the near and far stars, the ap- proximate distance of a few of them has been deter- mined. The nearest one is the brightest star in the Centaur, seldom visible in our northern latitudes, which has a parallax of about one second. The next nearest is No. 61 in the Swan, or 61 Cygni, having a parallax of 0".34:. Approximate measurements have been made on Sirius, Capella, the Pole Star, etc., about eighteen in all. The distances are immense : only the swiftest agents can traverse them. If our earth were suddenly to dis- solve its allegiance to the king of day, and attempt a flight to the North Star, and should maintain its flight of one thousand miles a minute, it would fly away to- ward Polaris for thousands upon thousands of years, till a million years had passed away, before it reached that northern dome of the distant sky, and gave its new alle- giance to another sun. The sun it had left behind it would gradually diminish till it was small as Arcturus, then small as could be discerned by the naked eye, until at last it would finally fade out in utter darkness long before the new sun was reached. Light can trav- erse the distance around our earth eight times in one second. It comes in eight minutes from the sun, but it takes three and a quarter years to come from Alpha

USES OF ASTRONOMY. 73

Centauri, seven and a quarter years from 61 Cygni, and forty-five years from the Polar Star.

Sometimes it happens that men steer along a lee shore, dependent for direction on Polaris, that light- house in the sky. Sometimes it has happened that men have traversed great swamps by night when that star was the light-house of freedom. In either case the exigency of life and liberty was provided for forty-five years before by a Providence that is divine.

We do not attempt to name in miles these enormous distances ; we must seek another yard-stick. Our astro- nomical unit and standard of measurement is the dis- tance-of the earth from the sun — 92,500,000 miles. This is the golden reed with which we measure the celestial city. Thus, by laying down our astronomi- cal unit 226,000 times, we measure to Alpha Centauri, more than twenty millions of millions of miles. Doubt- less other suns are as far from Alpha Centauri and each other as that is from ours.

Stars are not near or far according to their brightness. 61 Cygni is a telescopic star, while Sirius, the brightest star in the heavens, is twice as far away from us. One star differs from another star in intrinsic glory.

The highest testimonies to the accuracy of these ce- lestial observations are found in the perfect predictions of eclipses, transits of planets over the sun, occultation of stars by the moon, and those statements of the Nau- tical Almanac that enable the sailor to know exactly where he is on the pathless ocean by the telling of the stars : " On the trackless ocean this book is the mariner's trusted friend and counsellor ; daily and nightly its revelations bring safety to ships in all plirts of the

74 CELESTIAL MEASUREMENTS.

world. It is something more than a mere book ; it is an ever-present manifestation of the order and harmony of the universe."

Another example of this wonderful accuracy is found in tracing the asteroids. Within 200,000,000 or 300,000,000 miles from the sun, the two hundred and one (Sep- tember, 1879,) minute bodies that have been already discovered move in paths very nearly the same — indeed two of them traverse the same orbit, being one hundred and eighty degrees apart ; — they look alike, yet the eye of man in a few observations so determines the curve of each orbit, that one is never mistaken for another. But astronomy has higher uses than fixing time, estab- lishing landmarks, and guiding the sailor. It greatly quickens and enlarges thought, excites a desire to know, leads to the utmost exactness, and ministers to adora- tion and love of the Maker of the innumerable suns.

THE SUN.

"And God made two great lights; the greater light to rule the day, and the lesser light to rule the night : he made the stars also. " — Gen. i. 16.

'* It is perceived that the sun of the world, with all its essence, which is heat and light, flows into every tree, and into every shrub and flower, and into every stone, mean as well as precious ; and that every object takes its portion from this common influx, and that the sun does not divide its light and heat, and dispense a part to this and a part to that. It is simi- lar with the sun of heaven, from which the Divjne love proceeds as heat, and the Divine wisdom as light; these two flow into human minds, as the heat and light of the sun of the world into bodies, and vivify them ac- cording to the quality of the minds, each of which takes from the com- mon influx as much as is necessary." — SWEDENBORG.

THE SUN. 77

Y.

THE SUN.

SUPPOSE we had stood on the dome of Boston State- house November 9th, 1872, on the night of the great conflagration, and seen the fire break out; seen the en- gines dash through the streets, tracking their path by their sparks; seen the fire encompass a whole block, leap the streets on every side, surge like the billows of a storm-swept sea ; seen great masses of inflammable gas rise like dark clouds from an explosion, then take fire in the air, and, cut off from the fire below, float like argosies of flame in space. Suppose we had felt the wind that came surging from all points of the compass to fan that conflagration till it was light enough a mile away to see to read the finest print, hot enough to de- compose the torrents of water that were dashed on it, making new fuel to feed the flame. Suppose we had seen this spreading fire seize on the whole city, extend to its environs, and, feeding itself on the very soil, lick up Worcester with its tongues of flame — Albany, New York, Chicago, St. Louis, Cincinnati — and crossing the plains swifter than a prairie fire, making each peak of the Kocky Mountains hold up aloft a separate torch of flame, and the Sierras whiter with heat than they ever were with snow, the waters of the Pacific resolve into their constituent elements of oxygen and hydrogen, and

78 ^ SOLAR SYSTEM.

burn with unquenchable fire! We withdraw into the air, and see below a world on fire. All the prisoned powers have burst into intensest activity. Quiet breezes have become furious tempests. Look around this flam- ing globe — on fire above, below, around — there is noth- ing but fire. Let it roll beneath us till Boston comes round again. No ember has yet cooled, no spire of flame has shortened, no surging cloud has been quiet- ed. Not only are the mountains still in flame, but other ranges burst up out of the seething sea. There is no place of rest, no place not tossing with raging flame ! Yet all this is only a feeble figure of the great burn- ing sun. It is but the merest hint, a million times too insignificant.

The sun appears small and quiet to us because we are so far away. Seen from the various planets, the rela- tive size of the sun appears as in Fig. 30. Looked for from some of the stars about us, the sun could not be seen at all. Indeed, seen from the earth, it is not al- ways the same size, because the distance is not always the same. If we represent the size of the sun by one thousand on the 23d of September or 21st of March, it would be represented by nine hundred and sixty-seven on the 1st of July, and by one thousand and thirty-four on the 1st of January.

We sometimes speak of the sun as having a diameter of 860,000 miles. We mean that that is the extent of the body as seen by the eye. But that is a small part of its real diameter. So we say the earth has an equa- torial diameter of 7925£ miles, and a polar one of 7899. But the air is as much a part of the earth as the rocks are. The electric currents are as much a part of the

THE SUN.

79

earth as the ores and mountains they traverse. What the diameter of the earth is, including these, no man can tell. We used to say the air extended forty-five miles, but we now know that it reaches vastly farther. So of

. 30.— Relative Size of Sun as seen from Different Planets.

the sun, we might almost say that its diameter is infi- nite, for its light and heat reach beyond our measure- ment. Its living, throbbing heart sends out pulsations, keeping all space full of its tides of living light.

80

A SOL Alt SYSTEM.

i£. 31.— Zodiacal Light.

We might say with evident truth that the far-off planets are a part of the sun, since the space they trav- erse is filled with the power of that controlling king; not only with light, but also with gravitating power.

But come to more ponderable matters. If we look

THE SUN. 81

into our western sky soon after sunset, on a clear, moon- less night in March or April, we shall see a dim, soft light, somewhat like the milky- way, often reaching, well defined, to the Pleiades. It is wedge-shaped, in- clined to the south, and the smallest star can easily be seen through it. Mairan and Cassini affirm that they have seen sudden sparkles and movements of light in it. All our best tests show the spectrum of this light to be continuous, and therefore reflected : which indicates that it is a ring of small masses of meteoric matter surround- ing the sun, revolving with it and reflecting its light. One bit of stone as large as the end of one's thumb, in a cubic mile, would be enough to reflect what light we see looking through millions of miles of it. Perhaps an eye sufficiently keen and far away would see the sun surrounded by a luminous disk, as Saturn is -with his rings. As it extends beyond the earth's orbit, if this be measured as a part of the sun, its diameter would be about 200,000,000 miles.

Come closer. When the sun is covered by the disk of the moon at the instant of total eclipse, observers are startled by strange swaying luminous banners, ghostly and weird, shooting in changeful play about the central darkness (Fig. 32). These form the corona. Men have usually been too much moved to describe them, and have always been incapable of drawing them in the short minute or two of their continuance. But in 1878 men travelled eight thousand miles, coining and return- ing, in order that they might note the three minutes of total eclipse in Colorado. Each man had his work as- signed to him, and he was drilled to attend to that and nothing else. Improved instruments were put into his

82 A SOLAR SYSTEM.

hands, so that the sun was made to do his own drawing and give his own picture at consecutive instants. Fig. 33 is a copy of a photograph of the corona of 1878, by

Fig. 32.— The Corona in 1858, Brazil.

Mr. Henry Draper. It showed much less changeability that year than common, it being very near the time of least sun-spot. The previous picture was taken near the time of maximum sun-spot.

It was then settled that the corona consists of re- flected light, sent to us from dust particles or meteor- oids swirling in the vast seas, giving new densities and

THE SUN. 83

rarities, and hence this changeful light. "Whether they are there by constant projection, and fall again to the sun, or are held by electric influence, or by force of or- bital revolution, we do not know. That the corona can- not be in any sense an atmosphere of any continuous gas, is seen f rom the fact that the comet of 1843, pass- ing within 93,000 miles of the body of the sun, was not burned out of existence as a comet, nor in any percepti-

i£. 33.— The Corona iu 1S78, Colorado.

ble degree retarded in its motion. If the sun's diameter is to include the corona, it will be from 1,260,000 to 1,460,000 miles.

84 A SOLAR SYSTEM.

Come closer still. At the instant of the totality of the eclipse red flames of most fantastic shape play along the edge of the moon's disk. They can be seen at any time by the use of a proper telescope with a spectro- scope attached. I have seen them with great distinct- ness and brilliancy with the excellent eleven-inch tele- scope of the Wesleyan University. A description of their appearance is best given in the language of Professor Young, of Princeton College, who has made these flames the object of most successful study. On September 7th, 1871, he was observing a large hydrogen cloud by the sun's edge. This cloud was about 100,000 miles long, and its upper side was some 50,000 miles above the sun's surface, the lower side some 15,000 miles. The whole had the appearance of being supported on pillars of fire, these seeming pillars being in reality hydrogen jets brighter and more active than the substance of the cloud. At half -past twelve, when Professor Young chanced to be called away from his observatory, there were no indications of any approaching change, except that one of the connecting stems of the southern extrem- ity of the cloud had grown considerably brighter and more curiously bent to one side ; and near the base of another, at the northern end, a little brilliant lump had developed itself, shaped much like a summer thunder- head.

But when Professor Young returned, about half an hour later, he found that a very wonderful change had taken place, and that a very remarkable process was act- ually in progress. "The whole thing had been literally blown to shreds," he says, " by some inconceivable up- rush from beneath. In place of the quiet cloud I had

Fig. 34.— Solar Prominences of Flaming Hydrogen.

THE SUN. 87

left, the air — if I may use the expression — was filled with the flying debris, a mass of detached vertical fusi- form fragments, each from ten to thirty seconds (i. e., from four thousand five hundred to thirteen thousand five hundred miles) long, by two or three seconds (nine hundred to thirteen hundred and fifty miles) wide- brighter, and closer together where the pillars had for- merly stood, and rapidly ascending. When I looked, some of them had already reached a height of nearly four minutes (100,000 miles); and while I watched them they arose with a motion almost perceptible to the eye, until, in ten minutes, the uppermost were more than 200,000 miles above the solar surface. This was ascertained by careful measurements, the mean of three closely accordant determinations giving 210,000 miles as the extreme altitude attained. I am particular in the statement, because, so far as I know, chromato- spheric matter (red hydrogen in this case) has never before been observed at any altitude exceeding five minutes, or 135,000 miles. The velocity of ascent, also — one hundred and sixty -seven miles per second — is considerably greater than anything hitherto recorded. * * * As the filaments arose, they gradually faded away like a dissolving cloud, and at a quarter past one only a few filmy wisps, with some brighter streamers low down near the chromatosphere, remained to mark the place. But in the mean while the little ' thunder-head' before alluded to had grown and developed wonder- fully into a mass of rolling and ever-changing flame, to speak according to appearances. First, it was crowd- ed down, as it were, along the solar surface ; later, it arose almost pyramidally 50,000 miles in height ; then

88 A SOLAR SYSTEM.

its suriimit was drawn down into long filaments and threads, which were most curiously rolled backward and forward, like the volutes of an Ionic capital, and finally faded away, and by half-past two had vanished like the other. The whole phenomenon suggested most forcibly the idea of an explosion under the great prominence, acting mainly upward, but also in all directions out- ward ; and then, after an interval, followed by a corre- sponding in-rush."

No language can convey nor mind conceive an idea of the fierce commotion we here contemplate. If we call these movements hurricanes, we must remember that what we use as a figure moves but one hundred miles an hour, while these move one hundred miles a second. Such storms of fire on earth, " coming down upon us from the north, would, in thirty seconds after they had crossed the St. Lawrence, be in the Gulf of Mexico, carrying with them the whole surface of the continent in a mass not simply of ruins but of glowing vapor, in which the vapors arising from the dissolution of the materials composing the cities of Boston, New York, and Chicago would be mixed in a single indis- tinguishable cloud." In the presence of these evident visions of an actual body in furious flame, we need hes- itate no longer in accepting as true the words of St. Peter of the time " in which the [atmospheric] heav- ens shall pass away with a great noise, and the ele- ments shall melt with fervent heat ; the earth also, and the works that are therein, shall be burned up."

This region of discontinuous flame below the corona is called the chromosphere. Hydrogen is the principal material of its upper part ; iron, magnesium, and other

THE SUX. 89

metals, soine of them as yet unknown on earth, but hav- ing a record in the spectrum, in the denser parts below. If these fierce fires are a part of the sun, as they as- suredly are, its diameter would be from 1,060,000 to 1,260,000 miles.

Let us approach even nearer. AVe see a clearly recog- nized even disk, of equal dimensions in every direction. This is the photosphere. We here reach some definite- ly measurable data for estimating its visible size. We already know its distance. Its disk subtends an angle of 32' 12".6, or a little more than half a degree. Three hundred and sixty such suns, laid side by side, would span the celestial arch from east to west with a half circle of light. Two lines drawn from our earth at the angle mentioned would be 860,000 miles apart at the distance of 92,500,000 miles. This., then, is the diameter of the visible and measurable part of the sun. It would require one hundred and eight globes like the earth in a line to measure the sun's diameter, and three hundred and thirty-nine, to be strung like the beads of a neck- lace, to encircle his waist. The sun has a volume equal to 1.245,000 earths, but being only one-quarter as dense, it has a mass of only 326.800 earths. It has seven hun- dred times the mass of all the planets, asteroids, and satellites put together. Thus it is able to control them all by its greater power of attraction.

Concerning the condition of the surface of the sun many opinions are held. That it is hot beyond all esti- mate is indubitable. Whether solid or gaseous we are not sure. Opinions differ: some incline to the first theory, others to the second ; some deem the sun com- posed of solid particles, floating in gas so condensed

90 A SOLAR SYSTEM.

by pressure and attraction as to shine like a solid. It has no sensible changes of general level, but has pro- digious activity in spots. These spots have been the objects of earnest and almost hourly study on the part of such men as Secchi, Lockyer, Faye, Young, and oth- ers, for years. But it is a long way off to study an ob- ject. No telescope brings it nearer than 200,000 miles. Theory after theory has been advanced, each one satis- factory in some points, none in all. The facts about the spots are these : They are most abundant on the two sides of the equator. They are gregarious, depressed below the surface, of vast extent, black in the centre, usually surrounded by a region of partial darkness, be- yond which is excessive light. They have motion of their own over the surface — motion rotating about an axis, upward and downward about the edges. They change their apparent shape as the sun carries them

Fig. 35.— Change in Spots as rotated across the Disk, showing Cavities.

across its disk by axial revolution, being narrow as they present their edges to us, and rounder as we look per- pendicularly into them (Fig. 35).

These spots are also very variable in number, some- times there being none for nearly two hundred days, and again whole years during which the sun is never with- out them. The period from maximum to maximum

THE SUN. 91

of spots is about eleven years. We might look for them again and again in vain this year (1878). They will be most numerous in 1882 and 1893. The cause of this periodicity was inferred to be the near approach of the enormous planet Jupiter, causing disturbance by its attraction. But the periods do not correspond, and the cause is the result of some law of solar action to us as yet unknown.

These spots may be seen with almost any telescope, the eye being protected by deeply colored glasses.

Until within one hundred years they were supposed to be islands of scoriae floating in the sea of molten matter. But they were depressed below the surface, and showed a notch when on the edge. Wilson origi- nated and Herschel developed the theory that the sun's real body was dark, cool, and habitable, and that the photosphere was a luminous stratum at a distance from the real body, with openings showing the dark spots below. Such a sun would have cooled off in a week, but would previously have annihilated all life below.

The solar spots being most abundant on the two sides of the equator, indicates their cyclonic character; the centre of a cyclone is rarefied, and therefore colder, and cold on the sun is darkness. M. Faye says : " Like our cyclones, they are descending, as I have proved by a special study of these terrestrial phenomena. They carry down into the depths of the solar mass the cooler materials of the upper layers, formed principally of hydrogen, and thus produce in their centre a decided extinction of light and heat as long as the gyratory movement continues. Finally, the hydrogen set free at the base of the whirlpool becomes reheated at this

92

A SOLAR SYSTEM.

great depth, and rises up tmnultuously around the whirl- pool, forming irregular jets, which appear above the chromosphere. These jets constitute the protuberances. The whirlpools of the sun, like those on the earth, are of all dimensions, from the scarcely visible pores to the enormous spots which we see from time to time. They

Fig. 36.— Solar Spot, by Langley.

have, like those of the earth, a marked tendency, first to increase and then to break up, and thus form a row of spots extending along the same parallel."

A spot of 20,000 miles diameter is quite small ; there was one 74,816 miles across, visible to the naked eye for a week in 1843. This particular sun-spot somewhat

THE SUN. 93

helped the Millerites. On the day of the eclipse, in 1858, a spot over 107,000 miles in extent was clearly seen. In such vast tempests, if there were ships built as large as the whole earth, they would be tossed like autumn leaves in an ocean storm.

The revolution of the sun carries a spot across its face in about fourteen days. After a lapse of as much more time, they often reappear on the other side, changed but recognizable. They often break out or disappear under the eye of the observer. They divide like a piece of ice dropped on a frozen pond, the pieces sliding off in every direction, or combine like separate floes driven together into a pack. Sometimes a spot will last for more than two hundred days, recognizable through six or eight revolutions. Sometimes a spot will last only half an hour.

The velocities indicated by these movements are in- credible. An up-rush and down-rush at the sides has been measured of twenty miles a second ; a side-rush or whirl, of one hundred and twenty miles a second. These tempests rage from a few days to half a year, traversing regions so wide that our Indian Ocean, the realm of storms, is too small to be used for comparison ; then, as they cease, the advancing sides of the spots approach each other at the rate of 20,000 miles an hour; they strike together, and the rising spray of fire leaps thou- sands of miles into space. It falls again into the in- candescent surge, rolls over mountains as the sea over pebbles, and all this for eon after eon without sign of exhaustion or diminution. All these swift succeeding Himalayas of lire, where one hundred worlds could be buried, do not usually prevent the sun's appearing to our far-off eyes as a perfect sphere.

94 A SOLAR SYSTEM.

What the Sun does for us.

To what end does this enormous power, this central source of power, exist? That it could keep all these gigantic forces within itself could not be expected. It is in a system where every atom is made to affect every other atom, and every world to influence every other. The Author of all lives only to do good, to send rain on the just and unjust, to cause his sun to rise on the evil and the good, and to give his spirit, like a perpet- ually widening river, to every man to profit withal.

The sun reaches his unrelaxing hand of gravitation to every other world at every instant. The tendency of every world is to fly off in a straight line. This ten- dency must be momentarily curbed, and the planet held in its true curve about the sun. These giant worlds must be perfectly handled. Their speed, amounting to seventy times as fast as that of a rifle -ball, must be managed. Each and every world ma}7 be said to be lifted momentarily and swung perpetually at arm's- length by the power of the sun.

The sun warms us. It would convey but a small idea of the truth to state how many hundreds of mill- ions of cubic miles of ice could be melted by the sun every second without quenching its heat; but, if any one has any curiosity to know, it is 287,200,000 cubic miles of ice per second.

We journey through space which has a temperature of 200° below zero ; but we live, as it were, in a con- servatory, in the midst of perpetual winter. We are roofed over by the air that treasures the heat, floored under by strata both absorptive and retentive of heat,

THE SUN. 95

and between the earth and air violets grow and grains ripen. The sun has a strange chemical power. It kisses the cold earth, and it blushes with flowers and matures the fruit and grain. We are feeble creatures, and the sun gives us force. By it the light winds move one-eighth of a mile an hour, the storm tifty miles, the hurricane one hundred. The force is as the square of the velocity. It is by means of the sun that the mer- chant's white-sailed ships are blown safely home. So the sun carries off the miasma of the marsh, the pollu- tion of cities, and then sends the winds to wash and cleanse themselves in the sea-spray. The water-falls of the earth turn machinery, and make Lowells and Man- chesters possible, because the sun lifted all that water to the hills.

Intermingled with these currents of air are the cur- rents of electric power, all derived from the sun. These have shown their swiftness and willingness to serve man. The sun's constant force displayed on the earth is equal to 543,000,000,000 engines of 400-horse power each, working day and night; and yet the earth receives onbT 2 3 s i 0*0 o o o o Part of the whole force of the sun.

Besides all this, the sun, with provident care, has made and given to us coal. This omnipotent worker has stored away in past ages an inexhaustible reservoir of his power which man may easily mine and direct, thus releasing himself from absorbing toil.

EXPERIMENTS.

Any one may see the spots on the sun who has a spy-glass. Darken the room and put the glass through an opening toward the sun, as shown in Fig. 37. The eye-piece should be drawn out about half an inch be-

96

A SOLAR SYSTEM.

yond its usual focusing for distant objects. The farther it is drawn, the nearer must \ve hold the screen for a perfect image.

By holding a paper near the eye-piece, the proper direction of the instru- ment may be discovered without injury to the eyes. By this means the sun can be studied from day to day, and its spots or the transits of Mer- cury and Venus shown to any number of spectators.

Fig. 37.— Holding Telescope to see the Sim's Spots.

First covering the eyes with very dark or smoked glasses, erect a disk of pasteboard four inches in diameter between you and the sun ; close one eye; stand near it, and the whole sun is obscured. Withdraw from it till the sun's rays just shoot over the edge of the disk on every side. Measure the distance from the eye to the disk. You will be able to de- termine the distance of the sun by the rule of three: thus, as four inches is to 860,000 miles, so is distance from eye to disk to distance from disk to the sun. Take such measurements at sunrise, noon, and sunset, and see the apparently differing sizes due to refraction.

VI,

THE PLANETS, AS SEEN FROM SPACE.

"He hangeth the earth upon nothing." — Job xxvi. 7.

5

"Let a power be delegated to a finite spirit equal to the projection of the most ponderous planet in its orbit, and, from an exhaustless magazine, let this spirit select his grand central orb. Let him with puissant arm locate it in space, and, obedient to his mandate, there let it remain forever fixed. He proceeds to select his planetary globes, which he is now re- quired to marshal in their appropriate order of distance from the sun. Heed well this distribution ; for should a single globe be misplaced, the divine harmony is destroyed forever. Let us admit that finite intelligence mav at length determine the order of combination ; the mighty host is arrayed in order. These worlds, like fiery coursers, stand waiting the command to fly. But, mighty spirit, heed well the grand step, ponder well the direction in which thou wilt launch each waiting world ; weigh well the mighty impulse soon to be given, for out of the myriads of direc- tions, and the myriads of impulsive forces, there comes but a single com- bination that will secure the perpetuity of your complex scheme. In vain does the bewildered finite spirit attempt to fathom this mighty depth. In vain does it seek to resolve the stupendous problem. It turns away, and while endued with omnipotent power, exclaims, ' Give to me infinite wis- dom, or relieve me from the impossible task!'" — O. M. MITCHEL, LL.D.

THE PLANETS, AS SEEN FROM SPACE. 99

VI.

THE PLANETS, AS SEEN FROM SPACE.

IF we were to go out into space a few millions of miles from either pole of the sun, and were endowed with wonderful keenness of vision, we should perceive certain facts, viz : That space is frightfully dark except when we look directly at some luminous body. There is no air to bend the light out of its course, no clouds or other objects to reflect it in a thousand directions. Every star is a brilliant point, even in perpetual sun- shine. The cold is frightful beyond the endurance of our bodies. There is no sound of voice in the absence of air, and conversation by means of vocal organs being impossible, it must be carried on by means of mind communication. We see below an unrevolving point on the sun that marks its pole. Ranged round in order are the various planets, each with its axis pointing in very nearly the same direction. All planets, except pos- sibly Venus, and all moons except those of Uranus and Neptune, present their equators to the sun. The direc- tion of orbital and axial revolution seen from above the North Pole would be opposite to that of the hands of a watch.

The speed of this orbital revolution must be propor- tioned to the distance from the sun. The attraction oi the sun varies inversely as the square of the distance.

100

A SOLAR SYSTEM.

Fig. 38.— Orbits and Comparative Sizes of the Planets.

It holds a planet with a certain power ; one twice as far off, with one-fourth that power. This attraction must be counterbalanced by centrifugal force ; great force from great speed when attraction is great, and small from less

THE PLANETS, AS SEEN FROM SPACE. 101

speed when attractive power is diminished by distance. Hence Mercury must go 29.5 miles per second — seven- ty times as fast as a rifle-ball that goes two-fifths of a mile in a second — or be drawn into the sun; while Neptune, seventy -five times as far off, and hence at- tracted only 3-5^ as much, must be slowed down to 3.4 miles a second to prevent its flying away from the fee- bler attraction of the sun. The orbital velocity of the various planets in miles per second is as follows :

Mercury 29.55

Venus 21.61

Earth 18.38

Mars..., .. 14.90

Jupiter 8.06

Saturn 5.95

Uranus 4.20

Neptune 3.36

Hence, while the earth makes one revolution in its year, Mercury has made over four revolutions, or pass- ed through four years ; the slower Neptune has made only y^f of one revolution.

The time of axial rotation which determines the length of the day varies with different planets. The periods of the four planets nearest the sun vary only half an hour from that of the earth, while the enor- mous bodies of Jupiter and Saturn revolve in ten and ten and a quarter hours respectively. This high rate of speed, and its resultant, centrifugal force, has aided in preventing these bodies from becoming as dense as they would otherwise be — Jupiter being only 0.24 as dense as the earth, and Saturn only 0.13. This extremely rapid revolution produces a great flattening at the poles. If Jupiter should rotate four times more rapidly than it does, it could not be held together compactly. As it is, the polar diameter is five thousand miles less than the equatorial : the difference in diameters produced by the

102 A SOLAR SYSTEM.

same cause on the earth, owing to the slower motion and smaller mass, being only twenty -six miles. The effect of this will be more specifically treated here- after.

The difference in the size of the planets is very -no- ticeable. If we represent the sun by a gilded globe two feet in diameter, we must represent Vulcan and Mercury by mustard-seeds; Venus, by a pea ; Earth, by another ; Mars, by one-half the size ; Asteroids, by the motes in a sunbeam ; Jupiter, by a small-sized orange ; Saturn, by a smaller one ; Uranus, by a cherry ; and Neptune, by one a little larger.

Apply the principle that attraction is in proportion to the mass, and a man who weighs one hundred and fifty pounds on the earth weighs three hundred and ninety-six on Jupiter, and only fifty-eight on Mars; while on the Asteroids he could play with bowlders for marbles, hurl hills like Milton's angels, leap into the fifth- story windows with ease, tumble over precipices without harm, and go around the little worlds in seven jumps.

The seasons of a planet are caused by the inclination of its axis to the plane of its orbit. In Fig. 39 the ro- tating earth is seen at A, with its northern pole turning in constant sunlight, and its southern pole in constant darkness ; everywhere south of the equator is more dark- ness than day, and hence winter. Passing on to B, the world is seen illum'inated equally on each side of the equator. Every place has its twelve hours' darkness and light at each revolution. But at C — the axis of the earth always preserving the same direction — the north- ern pole is shrouded in continual gloom. Every place

THE PLANETS, AS SEEN FROM SPACE. 105

north of the equator gets more darkness than light, and hence winter.

The varying inclination of the axes of the different planets gives a wonderful variety to their seasons. The sun is always nearly over the equator of Jupiter, and every place has nearly its five hours day and five hours night. The seasons of Earth, Mars, and Saturn are so much alike, except in length, that no comment is nec- essary. The ice-fields at either pole of Mars are ob- served to enlarge and contract, according as it is win- ter or summer there. Saturn's seasons are each seven and a half years long. The alternate darkness and light at the poles is fifteen years long.

But the seasons of Venus present the greatest anom- aly, if its assigned inclination of axis (75°) can be relied on as correct, which is doubtful. Its tropic zone extends nearly to the pole, and at the same time the winter at the other pole reaches the equator. The short period of this planet causes it to present the south pole to the sun only one hundred and twelve days after it has been scorching the one at the north. This gives two win- ters, springs, summers, and autumns to the equator in two hundred and twenty-five days.

If each whirling world should leave behind it a trail of light to mark its orbit, and our perceptions of form were sufficiently acute, we should see that these curves of light are not exact circles, but a little flattened into an ellipse, with the sun always in one of the foci. Hence each planet is nearer to the sun at one part of its orbit than another; that point is called the perihelion, and the farthest point aphelion. This eccentricity of orbit, or distance of the sun from the centre, is very small.

106 A SOLAR SYSTEM.

In the case of Venus it is only .007 of the whole, and in no instance is it more than .2, viz., that of Mercury. This makes the sun appear twice as large, bright, and hot as seen and felt on Mercury at its perihelion than at its aphelion. The earth is 3,236,000 miles nearer to the sun in our winter than summer. Hence the summer in the southern hemisphere is more intolerable than in the northern. But this eccentricity is steadily diminishing at a uniform rate, by reason of the perturbing influence of the other planets. In the case of some other planets it is steadily increasing, and, if it were to go on a suffi- cient time, might cause frightful extremes of tempera- ture; but Lalande has shown that there are limits at which it is said, " Thus far shalt thou go, and no far- ther." Then a compensative diminution will follow.

Conceive a large globe, to represent the sun, float- ing in a round pond. The axis will be inclined 7^° to the surface of the water, one side of the equator be 7-J° below the surface, and the other side the same distance above. Let the half -sub merged earth sail around the sun in an appropriate orbit. The surface of the water will be the plane of the orbit, and the water that reaches out to the shore, where the stars would be set, will be the plane of the ecliptic. It is the plane of the earth's orbit extended to the stars.

The orbits of all the planets do not lie in the same plane, but are differently inclined to the plane of the ecliptic, or the plane of the earth's orbit. Going out from the sun's equator, so as to see all the orbits of the planets on the edge, we should see them inclined to that of the earth, as in Fig. 40.

If the earth, and Saturn, and Pallas were lying at

THE PLANETS, AS SEEN FROM SPACE.

107

Fig. 40.— luclination of the Planes of Orbits.

right angles with the nodal line of their orbits, and in the same direction from the sun, and the outer bodies were to start in a direct line for the snn, they would not collide with the earth on their way ; but Saturn would pass 4,000,000 and Pallas 50,000,000 miles over our heads. From this same cause we do not see Yenus and Mercury make a transit across the disk of the sun at every revolution.

Fig. 41 shows a view of the orbits of the earth and

Fig. 41.— Inclination of Orbits of Venus and Earth. Nodal Line, D B.

Yenus seen not from the edge but from a position somewhat above. The point E, where Yenus crosses the plane of the earth's orbit, is called the ascending node. If the earth were at B when Yenus is at E, Yenus would be seen on the disk of the sun, making a transit. The same would be true if the earth were at D, and Yenus at the descending node F.

This general view of the flying spheres is full of in-

108 A SOLAR SYSTEM.

terest. While quivering themselves with thunderous noises, all is silent about them ; earthquakes may be struggling on their surfaces, but there is no hint of contention in the quiet of space. They are too distant from one another to exchange signals, except, perhaps, the fleet of asteroids that sail the azure between Mars and Jupiter. Some of these come near together, con- tinuing to fill each other's sky for days with brightness, then one gradually draws ahead. They have all phases for each other — crescent, half, full, and gibbous. These hundreds of bodies fill the realm where they are with inexhaustible variety. Beyond are vast spaces — cold, dark, void of matter, but full of power. Occasionally a little spark of light looms up rapidly into a world so huge that a thousand of our earths could not occupy its vast bulk. It swings its four or eight moons with per- fect skill and infinite strength ; but they go by and leave the silence unbroken, the darkness unlighted for years. Nevertheless, every part of space is full of pow- er. Nowhere in its wide orbit can a world find a place ; at no time in its eons of flight can it find an instant when the sun does not hold it in safety and life.

The Outlook from the Earth.

If we come in from our wanderings in space and take an outlook from the earth, we shall observe certain movements, easily interpreted now that we know the system, but nearly inexplicable to men who naturally supposed that the earth was the largest, most stable, and central body in the universe.

We see, first of all, sun, moon, and stars rise in the east, mount the heavens, and set in the west. As I

THE PLANETS, AS SEEN FROM SPACE. 109

revolve in my pivoted study-chair, and see all sides of the room — library, maps, photographs, telescope, and windows — I have no suspicion that it is the room that whirls ; but looking out of a car-window in a depot at another car, one cannot tell which is moving, whether it be his car or the other. In regard to the world, we have come to feel its whirl. We have noticed the pyr- amids of Egypt lifted to hide the sun ; the mountains of Hymettus hurled down, so as to disclose the moon that was behind them to the watchers on the Acropo- lis ; and the mighty mountains of Moab removed to re- veal the stars of the east. Train the telescope on any star; it must be moved frequently, or the world will roll the instrument away from the object. Suspend a cannon-ball by a fine wire at the equator ; set it vibrat- ing north and south, and it swings all day in precisely the same direction. But suspend it directly over the north pole, and set it swinging toward Washington ; in five hours after it is swinging toward the Sandwich Isl- ands; in twelve hours, toward Siam,in Asia; in eighteen hours, toward Rome, in Italy ; and in twenty-four, to- ward Washington again, not because it has changed the plane of its vibration, but because the earth has whirled beneath it, and the torsion of the wire has not been suf- ficient to compel the plane of the original direction to change with the turning of the earth. The law of in- ertia keeps it moving in the same direction. The same experimental proof of revolution is shown in a propor- tional degree at any point between the pole and the equator.

But the watchers on the Acropolis do not get turned over so as to see the moon at the same time every night.

110 A SOLAR SYSTEM.

We turn down our eastern horizon, but we do not find fair Luna at the same moment we did the night before. We are obliged to roll on for some thirty to fifty min- utes longer before we find the moon. It must be go- ing in the same direction, and it takes us longer to get round to it than if it were always in the same spot ; so we notice a star near the moon one night — it is 13° west of the moon the next night. The moon is going around

ii

* * * *+* * * » «•

» » »

•*

+ f ^

v

* *

+

Fig. 42.— Showing the Sun's Movement among the Stars.

the earth from west to east, and if it goes 13° in one day, it will take a little more than twenty-seven days to go the entire circle of 360°.

THE PLANETS, AS SEEN FROM SPACE. Ill

In our outlook we soon observe that we do not by our revolution come to see the same stars rise at the same hour every night. Orion and the Pleiades, our familiar friends in the winter heavens, are gone from the summer sky. Have they fled, or are we turned from them ? This is easily understood from Fig. 42.

When the observer on the earth at A looks into the midnight sky he sees the stars at E; but as the earth passes on to B, he sees those stars at E four minutes sooner every night ; and at midnight the stars at F are over his head. Thus in a year, by going around the sun, we have every star of the celestial dome in our mid- night sky. "We see also how the sun appears among the successive constellations. When we are at A, we see the sun among the stars at G ; but as we move to- ward B, the sun appears to move toward H. If we had observed the sun rise on the 20th of August, 18T6, we should have seen it rise a little before Regulus, and a little south of it, in such a relation as circle 1 is to the star in Fig. 43. By sunset the earth had moved enough to make the sun appear to be at circle 2, and by the next morning at cir- cle 3, at which time Regulus would rise before the sun. Thus the earth's motion seems to make the sun traverse a regular circle among the stars once a year : but it is not the sun that moves.

There are certain stars that have such irregular, un- certain, vagarious ways that they were called vagabonds, or planets, by the early astronomers. Here is the path of Jupiter in the year 1866 (Fig. 44). These bodies go forward for awhile, then stop, start aside, then retro-

112 A SOLAR SYSTEM.

grade, and go on again. Some are never seen far from the sun, and others in all parts of the ecliptic.

Pig. 44.

First see them as they stand to-day, as in Fig. 45. The observer stands on the earth at A. It has rolled over so far that he cannot see the sun ; it has set. But

Fig. 45 — Showing Position of Planets.

Yenus is still in sight; Jupiter is 45° behind Yenus, and Saturn is seen 90° farther east. When A has roll- ed a little farther, if he is awake, he will see Mars be- fore he sees the sun ; or, in common language, Yenus will set after, and Mars rise before the sun. All these bodies at near and far distances seem set in the starry dome, as the different stars seem in Fig. 42, p. 110.

The mysterious movements of advance and retreat are rendered intelligible by Fig. 46. The planet Mer- cury is at A, and, seen from the earth, B is located at #,

THE PLANETS, AS SEEN FROM SPACE. 113

on the background of the stars it seems to be among. It remains apparently stationary at a for some time, be-

Pig. 46. — Apparent Movements of an Inferior Planet.

cause approaching the earth in nearly a straight line. Passing D to C, it appears to retrograde among the stars to c ; remains apparently stationary for some time, then, in passing from C to E and A, appears to pass back among the stars to a. The progress of the earth, meanwhile, although it greatly retards the apparent mo- tion from A to C, greatly hastens it from C to A.

It is also apparent that Mercury and Yenus, seen from the earth, can never appear far from the sun. They must be just behind the sun as evening stars, or just before it as heralds of the morning. Venus is nev- er more than 47° from the sun, and Mercury never more than 30° ; indeed, it keeps so near the sun that very few people have ever seen the brilliant sparkler. Ob- serve how much larger the planet appears near the earth in conjunction at D than in opposition at E. Observe also what phases it must present, and how transits some- times take place.

1U

A SOLAR SYSTEM.

The movement of a superior planet, one whose orbit is exterior to the earth, is clear from Fig. 47. When the earth is at A and Mars at B, it will appear among the stars at C. When the earth is at D, Mars having moved more slowly to E, will have retrograded to F. It remains there while the earth passes on, in a line near- ly straight, from Mars to G ; then, as the earth begins to curve around the sun, Mars will appear to retraverse

Fig. 47. — Illustrating Movements of a Superior Planet.

the distance from F to C, and beyond. The farther the superior planet is from the earth the less will be the retrograde movement.

The reader should draw the orbits in proportion, and, remembering the relative speed of each planet, note the movement of each in different parts of their orbits.

To account for these most simple movements, the earlier astronomers invented the most complex and im- possible machinery. They thought the earth the centre, and that the sun, moon, and stars were carried about it, as stoves around a person to warm him. They thought these strange movements of the planets were accom- plished by mounting them on subsidiary eccentric wheels in the revolving crystal sphere. All that was

THE PLANETS, AS SEEN FROM SPACE. 115

needed to give them a right conception was a sinking of their world and themselves to an appropriate propor- tion, and an enlargement of their vision, to take in from an exalted stand-point a view of the simplicity of the perfect plan.

EXPERIMENTS.

Fix a rod, or tube, or telescope pointing at a star in the east or west, and the earth's revolution will be apparent in a moment, turning the tube away from the star. Point it at stars about the north pole, and those on one side will be found going in an opposite direction from those on the other, and very much slower than those about the equator. Any one can try the pendulum experiment who has access to some lofty place from which to suspend the ball. It was tried in Bunker Hill Monument a few years ago, and is to be tried in Paris, in the summer of 1879, with a seven -hundred -pound pendulum and a suspending wire seventy yards long. The advance and retrograde movements of planets can be illus- trated by twc persons walking around a centre and noticing the place where the person appears projected on the wall beyond.

PROCESSION OF STARS AND SOULS.

I STOOD upon the open casement, And looked upon the night,

And saw the westward-going stars Pass slowly out of sight.

Slowly the bright procession Went down the gleaming arch,

And my soul discerned the music Of the long triumphal march ;

Till the great celestial army, Stretching far beyond the poles,

Became the eternal symbol Of the mighty march of souls.

116 A SOLAR SYSTEM.

Onward, forever onward,

Red Mars led on his clan ; And the moon, like a mailed maiden,

Was riding in the van.

And some were bright in beauty,

And some were faint and small, But these might be, in their great heights,

The noblest of them all.

Downward, forever downward,

Behind earth's dusky shore, They passed into the unknown night —

They passed, and were no more.

No more ! oh, say not so !

And downward is not just ; For the sight is weak and the sense is dim

That looks through heated dust.

The stars and the mailed moon, Though they seem to fall and die,

Still sweep in their embattled lines An endless reach of sky.

And though the hills of Death

May hide the bright array, The marshalled brotherhood of souls

Still keeps its onward way.

Upward, forever upward,

I see their march sublime, And hear the glorious music

Of the conquerors of Time.

And long let me remember

That the palest fainting one May to diviner vision be

A bright and blazing sun.

THOMAS BUCHANAN READ.

VII.

SHOOTING-STARS, METEORS, AND COMETS.

" The Lord cast down great stones from heaven upon them unto Aze- kah, and they tied."— Joshua x. II.

A SWAKM OF MKTKOUS MKKTINO TUB EAUTII.

Their orbits are all parallel. Those coming in direct line to the eye appear as stars, having no motion. Those at one side of this line are seen in foreshortened perspective. Those farthest from the centre, other things being equal, appear longest. The centre, called the radiant point, of these November meteors is sit- uated in Leo ; that of the August meteors in Perseus. Over fifty such radiant points have been discovered. Over 30,000 meteors have been visible in au hour.

SHOOTING-STARS, METEORS, AND COMETS. 119

YIL SHOOTING-STARS, METEORS, AND COMETS.

BEFORE particularly considering the larger aggrega- tions of matter called planets or worlds as individuals, it is best to investigate a part of the solar system con- sisting of smaller collections of matter scattered every- where through space. They are of various densities, from a cloudlet of rarest gas to solid rock ; of various sizes, from a grain's weight to little worlds; of vari- ous relations to each other, from independent individ- uality to related streams millions of miles long. When they become visible they are called shooting-stars, which are evanescent star-points darting through the upper air, leaving for an instant a brilliant train ; meteors, sudden lights, having a discernible diameter, passing over a large extent of country, often exploding with violence (Fig. 48), and throwing down upon the earth aerolites ; and comets, vast extents of ghostly light, that come we know not whence and go we know not whither. All these forms of matter are governed by the same laws as the worlds, and are an integral part of the solar system — a part of the unity of the universe.

Every one has seen the so-called shooting - stars. They break out with a sudden brilliancy, shoot a few degrees with quiet speed, and are gone before we can say, " See there !" The cause of their appearance, the

120 A SOLAR SYSTEM.

conversion of force into heat by their contact with our atmosphere, has been already explained. Other facts remain to be studied. They are found to appear about seventy - three miles above the earth, and to disappear

Fig. 48.— Explosion of a Boiide.

about twenty miles nearer the surface. Their average velocity, thirty -five, sometimes rises to one hundred miles a second. They exhibit different colors, accord- ing to their different chemical substances, which are consumed. The number of them to be seen on differ- ent nights is exceedingly variable ; sometimes not more

SHOOTING-STARS, METEORS, AND COMETS. 121

than five or six an hour, and sometimes so many that a man cannot count those appearing in a small section of sky. This variability is found to be periodic. There are everywhere in space little meteoric masses of mat- ter, from the weight of a grain to a ton, and from the density of gas to rock. The earth meets 7,500,000 little bodies every day — there is collision — the little meteoroid gives out its lightning sign of extinction, and, consumed in fervent heat, drops to the earth as gas or dust. If we add the number light enough to be seen by a telescope, they cannot be less than 400,000,000 a day. Everywhere we go, in a space as large as that occupied by the earth and its atmosphere, there must be at least 13,000 bodies— one in 20,000,000 cubic miles — large enough to make a light visible to the naked eye, and forty times that number capable of revealing themselves to telescopic vision. Professor Peirce is

about to publish, as the startling result of his investi- gations, " that the heat which the earth receives direct- ly from meteors is the same in amount which it re- ceives from the sun by radiation, and that the sun re- ceives five-sixths of its heat from the meteors that fall upon it."

122

A SOLAR SYSTEM.

In 1T83 Dr. Schmidt was fortunate enough to have a telescopic view of a system of bodies which had turn- ed into meteors. These were two larger bodies fol- lowed by several smaller ones, going in parallel lines till they were extinguished. They probably had been revolving about each other as worlds and satellites be-

Fig. 50.— Saiita Rosa Aerolite.

fore entering our atmosphere. It is more than prob- able that the earth has many such bodies, too small to be visible, revolving around it as moons.

Aerolites.

Sometimes the bodies are large enough to bear the heat, and the unconsurned centre comes to the earth.

SHOOTING-STARS, METEORS, AND COMETS. 123

Their velocity has been lessened by the resisting air, and the excessive heat diminished. Still, if found soon after their descent, they are too hot to be handled. These are called aerolites or air-stones. There was a fall in Iowa, in February, 1875, from which fragments amounting to five hundred pounds weight were se- cured. On the evening of December 21st, 1876, a me- teor of unusual size and brilliancy passed over the states of Kansas, Missouri, Illinois, Indiana, and Ohio. It was first seen in the western part of Kansas, at an altitude of about sixty miles. In crossing the State of Missouri it began to explode, and this breaking up con- tinued while passing Illinois, Indiana, and Ohio, till it consisted of a large flock of brilliant balls chasing each other across the sky, the number being variously esti- mated at from twenty to one hundred. It was accom- panied by terrific explosions, and was seen along a path of not less than a thousand miles. "When first seen in Kansas, it is said to have appeared as large as the full moon, and with a train from twenty-five to one hun- dred feet long. Another, very similar in appearance and behavior, passed over a part of the same course in February, 1879. At Laigle, France, on April 26th, 1803, about one o'clock in the day, from two to three thou- sand fell. The largest did not exceed seventeen pounds weight. One fell in Weston, Connecticut, in 1807, weighing two hundred pounds. A very destructive shower is mentioned in the book of Joshua, chap. x. ver. 11.

These bodies are not evenly distributed through space. In some places they are gathered into systems which circle round the sun in orbits as certain as those of the

124: A SOLAR SYSTEM.

planets. The chain of asteroids is an illustration of meteoric bodies on a large scale. They are hundreds in number — meteors are millions. They have their region of travel, and the sun holds them and the giant Jupiter by the same power. The Power that cares for a world cares for a sparrow. If their orbit so lies that a planet passes through it, and the planet and the me- teors are at the point of intersection at the same time, there must be collisions, and the lightning signs of ex- tinction proportioned to the number of little bodies in a given space.

It. is demonstrated that the earth encounters more than one hundred such systems of meteoric bodies in a single year. It passes through one on the 10th of Au- gust, another on the llth of November. In a certain part of the first there is an agglomeration of bodies suf- ficient to become visible as it approaches the sun, and this is known as the comet of 1862 ; in the second is a similar agglomeration, known as Temple's comet. It is repeating the same thing to say that meteoroids follow in the train of the comets. The probable orbit of the No- vember meteors and the comet of 1866 is an exceeding- ly elongated ellipse, embracing the orbit of the earth at one end and a portion of the orbit of Uranus at the other (Fig. 51). That of the August meteors and the comet of 1862 embraces the orbit of the earth at one end, and thirty per cent, of the other end is beyond the orbit of Neptune.

In January, 1846, Biela's comet was observed to be divided. At its next return, in 1852, the parts were 1,500,000 miles apart. They could not be found on their periodic returns in 1859, 1865, and 1872 ; but it

SHOOTING-STARS, METEORS, AND COMETS. 125

should have crossed the earth's orbit early in Septem- ber, 1872. The earth itself would arrive at the point of crossing two or three months later. If the law of revo- lution held, we might still expect to find some of the trailing meteoroids of the comet not gone by on our ar-

ofiJLLL

VHAW/.S

Fig. 51.— Orbit of the November Meteors aud the Comet of 1866.

rival. It was shown that the point of the earth that would strike them would be toward a certain place in the constellation of Andromeda, if the remains of the di- luted comet were still there. The prediction was veri- fied in every respect. At the appointed time, place,

126 A SOLAR SYSTEM.

and direction, the streaming lights were in our sky. That these little bodies belonged to the original comet none can doubt. By the perturbations of planetary at- traction, or by different original velocities, a comet may be lengthened into an invisible stream, or an invisible stream agglomerated till it is visible as a comet.

Comets.

Comets will be most easily understood by the fore- going considerations. They are often treated as if they were no part of the solar system ; but they are under the control of the same laws, and owe their ex- istence, motion, and continuance to the same causes as Jupiter arid the rest of the planets. They are really planets of wider wandering, greater ellipticity, and less density. They have periodic times less than asteroids, and fifty times as great as Neptune. They are little clouds of gas or meteoric matter, or both, darting into the solar system from every side, at every angle with the plane of the ecliptic, becoming luminous with re- flected light, passing the sun, and returning again to outer darkness. Sometimes they have no tail, having a nucleus surrounded by nebulosity like a dim sun with zodiacal light; sometimes one tail, sometimes half a dozen. These follow the comet to perihelion, and pre- cede it afterward (Fig. 52).- The orbits of some comets are enormously elongated ; one end may lie inside the earth's orbit, and the other end be as far beyond Nep- tune as that is from the sun. Of course only a small part of such a curve can be studied by us : the comet is visible only when near the sun. The same curve around the sun may be an orbit that will bring it back again,,

SHOOTING-STARS, METEORS, AND COMETS. 127

Fig. 52. — Aspects of Remarkable Comets.

or one that will carry it off into infinite space, never to return. One rate of speed on that curve indicates an elliptical orbit; it returns; a greater rate of speed in- dicates that it will take a parabolic orbit, and never return. The exact rate of speed is exceedingly difficult to determine; hence it cannot be confidently asserted that any comet ever visible will not return. They may all belong to the solar system ; but some will certainly be gone thousands of years before their fiery forms will greet the watchful eyes of dwellers on the earth. A comet that has an elliptic orbit may have it changed to

128 A SOLAR SYSTEM.

parabolic by the accelerations of its speed, by attracting planets; or a parabolic comet may become elliptic, and so permanently attracted to the system by the retarda- tions of attracting bodies. A comet of long period may be changed to one of short period by such attraction, or vice versa. Orbits may be changed without affecting speed. The number of comets, like that of meteor streams, is exceedingly large. Five hundred have been visible to the naked eye since the Christian era. Two hun- dred have been seen by telescopes invented since their invention. Some authorities estimate the number be- longing to our solar system by millions ; Professor Peirce says more than five thousand millions.

Famous Comets.

The comet of 1680 is perhaps the one that appeared in A.D. 44, soon after the death of Julius Caesar, also in the reign of Justinian, A.D. 531, and in 1106. This is not determined by any recognizable resemblance. It had a tail 70° long; it was not all arisen when its head reached the meridian. It is possible, from the shape of its orbit, that it has a periodic time of nine thousand years, or that it may have a parabolic orbit, and never return. Observations taken two hundred years ago have not the exactness necessary to determine so delicate a point.

On August 19th, 1682, Halley discovered a comet which he soon declared to be one seen by Kepler in 1607. Looking back still farther, he found that a com- et was seen in 1531 having the same orbit. Still far- ther, by the same exact period of seventy-five years, he found that it was the same comet that had disturbed

SHOOTING-STARS, METEORS, AND COMETS. 129

the equanimity of Pope Calixtus in 1456. Calculations were undertaken as to the result of all the accelerations and retardations by the attractions of all the planets for the next seventy-five years. There was not time to fin- ish all the work ; but a retardation of six hundred and eighteen days was determined, with a possible error of thirty days. The comet actually came to time within thirty-three days, on March 12th, 1759. Again its re- turn was calculated with more laborious care. It came to time and passed the sun within three days of the pre- dicted time, on the 16th of November, 1835. It passed from sight of the most powerful telescopes the follow- ing May, and has never since been seen by human eye. But the eye of science sees it as having passed its aphelion beyond the orbit of Neptune in 1873, and is already hastening back to the warmth and light of the sun. It will be looked for in 1911 ; and there is good hope of predicting, long before it is seen, the time of its perihelion within a day.

Bieltfs lost Comet. — This was a comet with a periodic time of six years and eight months. It was observed in January, 1846, to have separated into two parts of unequal brightness. The lesser part grew for a month until it equalled the other, then became smaller and disappeared, while the other was visible a month longer. At disappearance the parts were 200,000 miles asunder. On its next return, in 1852, the parts were 1,500,000 miles apart ; sometimes one was brighter and sometimes the other ; which was the fragment and which was the main body could not be recognized. They vanished in September, 1852, and have never been seen since. Three revolutions have been made since that time, but no

6*

130 A SOLAR SYSTEM.

trace of it could be discovered. Probably the same in- fluence that separated it into parts, separated the par- ticles till too thin and tenuous to be seen. There is ground for believing that the earth passed through a part of it, as before stated under the head of meteors.

The Great Comet of 184:3 passed nearer the sun than any known body. It almost grazed the sun. If it ever returns, it will be in A.D. 2373.

Donates Comet of 1858. — This was one of the most magnificent of modern times. During the first three months it showed no tail, but from August to October it had developed one forty degrees in length. Its period is about two thousand years. Every reader remembers the comet of the summer of 1874.

Enckds Comet. — This comet has become famous for its supposed confirmation of the theory that space was filled with a substance infinitely tenuous, which resisted the passage of this gaseous body in an appreciable de- gree, and in long ages would so retard the motion of all the planets that gravitation would draw them all one by one into the sun. We must not be misled by the term retardation to suppose it means behind time, for a retarded body is before time. If its velocity is dimin- ished, the attraction of the sun causes it to take a small- er orbit, and smaller orbits mean increased speed — hence the supposed retardation would shorten its periodic time. This comet was thought to be retarded two and a half hours at each revolution. If it was, it would not prove the existence of the resisting medium. Other causes, unknown to us, might account for it. Subse- quent and more exact calculations fail to find any re- tardations in at least two revolutions between 1865 and

SHOOTING-STARS, METEORS, AND COMETS. 131

1871. Indications point to a retardation of one and a half hours both before and since. But such discrep- ancy of result proves nothing concerning a resisting medium, but rather is an argument against its existence. Besides, Faye's comet, in four revolutions of seven years each, shows no sign of retardation.

The truth may be this, that a kind of atmosphere ex- ists around the sun, perhaps revealed by the zodiacal light, that reaches beyond where Encke's comet dips in- side the orbit of Mercury, and thus retards this body, but does not reach beyond the orbit of Mars, where Faye's comet wheels and withdraws.

Of what do Comets consist?

The unsolved problems pertaining to comets are very numerous and exceedingly delicate. Whence come they ? Why did they not contract to centres of nebu- lae? Are there regions where attractions are balanced, and matter is left to contract on itself, till the move- ments of suns and planets adds or diminishes attrac- tive force on one side, and so allows them to be drawn slowly toward one planet, and its sun, or another? There is ground for thinking that the comet of 1866 and its train of meteors, visible to us in November, was thus drawn into our system by the planet Uranus. In- deed, Leverrier has conjecturally fixed upon the date of A.D. 128 as the time when it occurred; but another and closer observation of its next return, in 1899, will be needed to give confirmation to the opinion. Our sun's authority extends at least half-way to the nearest fixed star, one hundred thousand times farther than the orbit of the earth. Meteoric and cometary matter ly-

132 A SOLAR SYSTEM.

ing there, in a spherical shell about the solar system, balanced between the attraction of different suns, final- ly feels the power that determines its destiny toward our sun. It would take 167,000,000 years to come thence to our system.

The conditions of matter with which we are acquaint- ed do not cover all the ground presented by these mys- terious visitors. We know a gas sixteen times as light as air, but hydrogen is vastly too heavy and dense ; for we see the faintest star through thousands of miles of cometary matter; we know that water may become cloudy vapor, but a little of it obscures the vision. Into what more ethereal, and we might almost say spir- itual, forms matter may be changed we cannot tell. But if wre conceive comets to be only gas, it would expand indefinitely in the realms of space, where there is no force of compression but its own. We might say that comets are composed of small separate masses of mat- ter, hundreds of miles apart; and, looking through thou- sands of miles of them, we see light enough reflected from them all to seem continuous. Doubtless that is sometimes the case. But the spectroscope shows anoth- er state of things: it reveals in some of these comets an incandescent gas — usually some of the combinations of carbon. The conclusion, then, naturally is that there are both gas and small masses of matter, each with an orbit of its own nearly parallel to those of all the others, and that they afford some attraction to hold the mass of intermingled and confluent gas together. Our best judgment, then, is that the nucleus is composed of sepa- rate bodies, or matter in a liquid condition, capable of being vaporized by the heat of the sun, and driven off,

SHOOTING-STARS, METEORS, AND COMETS. 133

as steam from a locomotive, into a tail. Indications of this are found in the fact that tails grow smaller at suc- cessive returns, as the matter capable of such vaporiza- tion becomes condensed. In some instances, as in that of the comet of 1843, the head was diminished by the manufacture of a tail. On the other hand, Professor Peirce showed that the nucleus of the comets of 1680, 1843, and 1858 must have had a tenacity equal to steel, to prevent being pulled apart by the tidal forces caused by its terrible perihelion sweep around the sun.

It is likely that there are great varieties of condition in different comets, and in the same comet at times. We see them but a few days out of the possible millions of their periodic time ; we see them only close to the sun, under the spur of its tremendous attraction and terrible heat. This gives us ample knowledge of the path of their orbit and time of their revolution, but little ground for judgment of their condition, when they slowly round the uttermost cape of their far-voyaging, in the terrible cold and darkness, to commence their homeward flight. The unsolved problems are not all in the distant sun and more distant stars, but one of them is carried by us, sometimes near, sometimes far off ; but our acquaintance with the possible forms and conditions of matter is too limited to enable us to mas- ter the difficulties.

WiU Comets strike the Earth ?

Very likely, since one or two have done so within a recent period. What will be the effect ? That depends on circumstances. There is good reason to suppose we passed through the tail of a comet in 1861, and the only

134 A SOLAR SYSTEM.

observable effect was a peculiar phosphorescent mist. If the comet were composed of small meteoric masses a brilliant shower would be the result. But if we fairly encountered a nucleus of any considerable mass and so- lidity, the result would be far more serious. The mass of Donati's comet has been estimated by M. Faye to be inrumr of that of the earth. If this amount of matter were dense as water, it would make a globe five hun- dred miles in diameter; and if as dense as Professor Peirce proved the nucleus of this comet to be, its im- pact with the earth would develop heat enough to melt and vaporize the hardest rocks. Happily there is little fear of this: as Professor Newcomb says, "So small is the earth in comparison with celestial space, that if one were to shut his eyes and fire at random in the air, the chance of bringing down a bird would be better than that of a comet of any kind striking the earth." Be- sides, we are not living under a government of chance, but under that of an Almighty Father, who upholdeth all things by the word of his power; and no world can come to ruin till he sees that it is best.

THE PLANETS AS INDIVIDUALS.

"Through faith we understand that the worlds [plum I J were framed by the word of God, so that things which are seen were not made of tilings which do appear." — Heb. xi. 3.

" O rich and various man ! thou palace of sight and sound, carrying in thy senses the morning, and the night, and the unfathomable galaxy ; in thy brain the geometry of the city of God ; in thy heart the power of love, and the realms of right and wrong. An individual man is a fruit which it costs all the foregoing ages to form and ripen. He is strong, not to do but to live ; not in his arms, but in his heart ; not as an agent, but as a fact." — EMERSON.

THE PLANETS AS INDIVIDUALS. 137

VIII. THE PLANETS AS INDIVIDUALS.

How many bodies there may be revolving about the sun we have no means to determine or arithmetic to express. When the new star of the American Repub- lic appeared, there were but six planets discovered. Since then three regions of the solar system have been explored with wonderful success. The outlying realms beyond Saturn yielded the planet Uranus in 1781, and Neptune in 1846. The middle region between Jupiter and Mars yielded the little planetoid Ceres in 1801, Pallas in 1802, and one hundred and ninety others since. The inner region between Mercury and the sun is of necessity full of small meteoric bodies ; the ques- tion is, are there any bodies large enough to be seen ?

The same great genius of Leverrier that gave us Nep- tune from the observed perturbations of Uranus, point- ed out perturbations in Mercury that necessitated either a planet or a group of planetoids between Mercury and the sun. Theoretical astronomers, aided by the fact that no planet had certainly been seen, and that all as- serted discoveries of one had been by inexperienced ob- servers, inclined to the belief in a group, or that the dis- turbance was caused by the matter reflecting the zodi- acal light.

When the total eclipse of the sun occurred in 1878,

138 A SOLAR SYSTEM.

astronomers were determined that the question of the existence of an intra-mercurial planet should be settled. Maps of all the stars in the region of the sun were care- fully studied, sections of the sky about the sun were as- signed to different observers, who should attend to noth-

O '

ing but to look for a possible planet. It is now con- ceded that Professor Watson and Lewis Swift, the fa- mous comet-finder, each discovered two small bodies — four in all — within the orbit of Mercury. Instead of one Vulcan there are doubtless many planetoids. One was seen during the eclipse of January llth, 1880. They are about 13,000,000 miles from the sun, and make a revolution in about twenty days.

MERCURY.

The swift messenger of the gods ; sign $, his caducens.

Distance from the sun, 35,750,000 miles. Diameter, 2992 miles. Orbital revolution, 87.97 days. Orbital velocity, 1773 miles per minute. Axial revolution, 24h. 5m.

Mercury shines with a white light nearly as bright as Sirius ; is always near the horizon. When nearly be- tween us and the sun, as at D (Fig. 46, p. 113), its illu- minated side nearly opposite to us, we, looking from E, see only a thin crescent of its light. When it is at its greatest angular distance from the sun, as A or C, we see it illuminated like the half-moon. When it is be- yond the sun, as at E, we see its whole illuminated face like the full-moon.

The variation of its apparent size from the varying distance is very striking. At its extreme distance from the earth it subtends an angle of only five seconds ; near- est to us, an angle of twelve seconds. Its distance from the earth varies nearly as one to three, and its apparent size in the inverse ratio.

THE PLANETS AS INDIVIDUALS. 139

When Mercury comes between the earth and the sun, near the line where the planes of their -orbits cut each other by reason of their inclination, the dark body of Mercury will be seen on the bright surface of the sun. This is called a transit. If it goes across the centre of the sun it may consume eight hours. It goes 100,000 miles an hour, and has 860,000 miles of disk to cross. The transit of 1878 occupied seven and a half hours. The transits for the remainder of the century will occur :

November Ttl) 1881

May 9th 1891

November 10th 1894

November 4th 1901

VENUS.

Goddess of beauty; its sign $, a mirror.

Distance from the sun, 66,750,000 miles. Diameter, 7660 miles. Orbital Velocity, 1296 miles per minute. Axial rev- olution, 23h, 21m. Orbital revolution, 224.7 days.

This brilliant planet is often visible in the daytime. I was once delighted by seeing Venus looking down, a little after mid-day, through the open space in the dome of the Pantheon at Rome. It has never since seemed to me as if the home of all the gods was deserted. Phoebus, Diana, Yenus and the rest, thronged through that open upper door at noon of night or day. Arago relates that Bonaparte, upon repairing to Luxemburg when the Directory was about to give him a fete, was much surprised at seeing the multitude paying more attention to the heavens above the palace than to him or his brilliant staff. Upon inquiry, he learned that these curious persons were observing with astonishment a star which they supposed to be that of the conqueror of Italy. The emperor himself was not indifferent when

140 A SOLAR SYSTEM.

his piercing eye caught the clear lustre of Venus smil- ing upon him at rnid-day.

This unusual brightness occurs when Yenus is about five weeks before or after her inferior conjunction, and also nearest overhead by being north of the sun. This last circumstance occurs once in eight years, and came on February 16th, 1878.

Venus may be as near the earth as 22,000,000 miles, and as far away as 160,000,000. This variation of its distances from the earth is obviously much greater than that of Mercury, and its consequent apparent size much more changeable. Its greatest and least apparent sizes are as ten and sixty-five (Fig. 53).

o ° o

O ^t^fer ^

C 3

f) ^^ d

• • *

Fig. 53 Phases of Venns, and Various Apparent Dimensions.

When Copernicus announced the true theory of the solar system, he said that if the inferior planets could be clearly seen they would show phases like the moon. When Galileo turned the little telescope he had made on Venus, he confirmed the prophecy of Copernicus. Desiring to take time for more extended observation, and still be able to assert the priority of his discovery, he published the following anagram, in which his dis- covery was contained :

THE EARTH. 141

" Haec immatnra a me jam frustra leguntur o. y." (These unripe things are now vainly gathered by me.)

He first saw Verms as gibbous ; a few months revealed it as crescent, and then he transposed his anagram into :

" Cynthiae figuras aemulatur mater amorum." (The mother of loves imitates the phases of Cynthia.)

Many things that were once supposed to be known concerning Yenus are not confirmed by later and better observations. Yenus is surrounded by an atmosphere so dense with clouds that it is conceded that her time of rotation and the inclination of her axis cannot be deter- mined. She revealed one of the grandest secrets of the universe to the first seeker ; showed her highest beauty to her first ardent lover, and has veiled herself from the prying eyes of later comers.

Florence has built a kind of shrine for the telescope of Galileo. By it he discovered the phases of Yenus, the spots on the sun, the mountains of the moon, the satellites of Jupiter, and some irregularities of shape in Saturn, caused by its rings. Galileo subsequently be- came blind, but he had used his eyes to the best pur- pose of any man in his generation.

THE EARTH.

Its sign ©.

Distance from the sun, 92,500,000 milea Diameter, polar, 7899 miles ; equatorial, 7925^ miles. Axial revolution, 23h. 56m. 4.09s.; orbital, 365.26. Orbital velocity per minute, 1102.8 miles.

Let us lift ourselves up a thousand miles from the earth. We see it as a ball hung upon nothing in emp- ty space. As the drop of falling water gathers itself

142 A SOLAR SYSTEM.

into a sphere by its own inherent attraction, so the earth gathers itself into a ball. Noticing closely, we

Fig. 54 — Earth and Moon in Space.

see forms of continents outlined in bright relief, and oceanic forms in darker surfaces. We see that its axis of revolution is nearly perpendicular to the line of light from the sun. One-half is always dark. The sunrise greets a new thousand miles every hour ; the glories of

AURORA BOREALIS. 143

the sunset follow over an equal space, 180° behind. We are glad that the darkness never overtakes the morning.

The Aurora Borealis.

While east and west are gorgeous with sunrise and sunset, the north is often more glorious with its aurora borealis. We remember that all worlds have weird

Fig. 55.— The Anrora as Waving Ctirtains.

and inexplicable appendages. They are not limited to their solid surfaces or their circumambient air. The sun has its fiery flames, corona, zodiacal light, and perhaps a finer kind of atmosphere than we know. The earth is

144: A SOLAR SYSTEM.

not without its inexplicable surroundings. It lias not only its gorgeous eastern sunrise, its glorious western sunset, high above its surface in the clouds, but it also has its more glorious northern dawn far above its clouds and air. The realm of this royal splendor is as yet an unconquered world waiting for its Alexander. There are certain observable facts, viz., it prevails mostly near the arctic circle rather than the pole ; it takes on vari- ous forms — cloud-like, arched, straight; it streams like banners, waves like curtains in the wind, is inconstant ; is either the cause or result of electric disturbance ; it is often from four hundred to six hundred miles above the earth, while our air cannot be over one hundred miles. It almost seems like a revelation to human eyes of those vast, changeable, panoramic pictures by which the inhabitants of heaven are taught.

Investigation has discovered far more mysteries than it has explained. It is possible that the same cause that produces sun-spots produces aurora in all space, visible in all worlds. If so, we shall see more abundant auroras at the next maximum of sun-spot, between 1880-84.

The Delicate Balance of Forces.

A soap-bubble in the wind could hardly be more flex- ible in form and sensitive to influence than is the earth. On the morning of May 9th, 1876, the earth's crust at Peru gave a few great throbs upward, by the action of expansive gases within. The sea fled, and returned in great waves as the land rose and fell. Then these waves fled away over the great mobile surface, and in less than five hours they had covered a space equal to half of Eu- rope. The waves ran out to the Sandwich Islands, six

THE EARTH. 14:5

thousand miles, at the rate of five hundred miles an hour, and arrived there thirty feet high. They not only sped on in straight radial lines, but, having run up the coast to California, were deflected away into the former series of waves, making the most complex undulations. Similar beats of the great heart of the earth have sent its pulses as widely and rapidly on previous occasions.

The figure of the earth, even on the ocean, is irregu- lar, in consequence of the greater preponderance of land — and hence greater density — in the northern hemi- sphere. These irregularities are often very perplexing in making exact geodetic measurements. The tendency of matter to fly from the centre by reason of revolu- tion causes the equatorial diameter to be twenty -six miles longer than the polar one. By this force the Mississippi River is enabled to run up a hill nearly three miles high at a very rapid rato. Its mouth is that distance farther from the centre of the earth than its source, when but for this rotation both points would be equally distant.

If the water became more dense, or if the world were to revolve faster, the oceans would rush to the equator, burying the tallest mountains and leaving polar regions bare. If the water should become lighter in a very slight degree, or the world rotate more slowly, the